Implement the two-fer exercise

This is meant to be right after hello-world introducing users to basic
programming constructs as well as TDD which will be used in all later
exercises.

Discussion regarding this is [here](exercism#548).

Closes exercism#757.

exercises/two-fer/canonical-data.json

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+{
+  "exercise": "two-fer",
+  "version": "1.0.0",
+  "cases": [
+    {
+      "description": "no name given",
+      "property": "twoFer",
+      "input": null,
+      "expected": "One for you, one for me."
+    },
+    {
+      "description": "a name given",
+      "property": "twoFer",
+      "input": "Alice",
+      "expected": "One for Alice, one for me."
+    },
+    {
+      "description": "another name given",
+      "property": "twoFer",
+      "input": "Bob",
+      "expected": "One for Bob, one for me."
+    }
+  ]
+}

exercises/two-fer/description.md

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+`Two-fer` or `2-fer` is short for two for one. One for you and one for me.
+
+This exercise expects you to create a sentence of the form:
+
+```
+"One for X, one for me."
+```
+
+When X is a name or "you".
+
+If the given name is "Alice", the result should be "One for Alice, one for me."
+If no name is given, the result should be "One for you, one for me."
+
+
+## Test-Driven Development
+
+As programmers mature, they eventually want to test their code.
+
+Here at Exercism we simulate [Test-Driven
+Development](http://en.wikipedia.org/wiki/Test-driven_development) (TDD), where
+you write your tests before writing any functionality. The simulation comes in
+the form of a pre-written test suite, which will signal that you have solved
+the problem.
+
+It will also provide you with a safety net to explore other solutions without
+breaking the functionality.
+
+### A typical TDD workflow on Exercism:
+
+1. Run the test file and pick one test that's failing.
+2. Write some code to fix the test you picked.
+3. Re-run the tests to confirm the test is now passing.
+4. Repeat from step 1.
+5. Submit your solution (`exercism submit /path/to/file`)
+
+## Instructions
+
+Submissions are encouraged to be general, within reason. Having said that, it's
+also important not to over-engineer a solution.
+
+It's important to remember that the goal is to make code as expressive and
+readable as we can. However, solutions to the hello-world exercise will not be
+reviewed by a person, but by rikki- the robot, who will offer an encouraging
+word.

exercises/two-fer/metadata.yml

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+---
+blurb: 'Create a sentence of the form "One for X, one for me."'
+source: "This is an exercise to introduce users to basic programming constructs, just after hello World."
+source_url: "https://en.wikipedia.org/wiki/Two-fer"