Add docs, more errors, fix tests, add examples.

[aybabtme]

Good morning!

• Add docs: I've written some docs for all the exported fields.
• Add examples: I replaced the example/resize.go file by Go examples, which are part of the test suite and show up in the docs.

See the docs + examples : http://godoc.org/github.com/aybabtme/mogrify-go

• More errors: replaced some calls that required nil-checks (not idiomatic in Go) by errors. I think more could be done for the cases where methods on gdImage panics for various operations that could be errors. However I'm not familiar enough so I didn't venture this way.
• Fix tests: the tests were passing an asset string argument to the asset(...) helper func, which was diligently ignoring it and always opening the same file.

r: @tobi
cc: @boourns
This PR includes commits I've found in @boourns fork, so this builds on top of it.