Add logging

[kittsville]

Thought I'd keep it simple with the first logging PR. We can add other info/debugging stuff as and when needed:

• Adds a logging library for proper management of log visibility
• Can now enable web logs with an env param
• Defaults to info level and only logs from gitkv itself
• Logs listening port/IP and loaded repos

Example log:

Kit:gitkv Kit$ cargo run -- --repo-root=/Users/Kit/Projects
   Compiling gitkv v0.1.0 (/Users/Kit/Projects/sandbox/gitkv/server)
    Finished dev [unoptimized + debuginfo] target(s) in 9.16s
     Running `target/debug/gitkv --repo-root=/Users/Kit/Projects`
[2019-04-15T13:35:30Z INFO  gitkv] Loaded Git repos: [Recibase, gitkv, anon, business-cat]
[2019-04-15T13:35:30Z INFO  gitkv] Listening on localhost:7791

[nathankleyn]

A suggestion on how to avoid some memory allocation and copying here by having a slightly different log and using std::fmt features! 👍

[nathankleyn]

Spoke to @kittsville on Slack quickly — just needs the unused import removed, then build will pass. LGTM! 👏

[nathankleyn]

LGTM! 👍

[codecov-io]

Codecov Report

Merging #30 into master will increase coverage by 0.09%.
The diff coverage is 80%.

@@           Coverage Diff           @@
##           master   #30      +/-   ##
=======================================
+ Coverage    69.9%   70%   +0.09%     
=======================================
  Files           3     3              
  Lines         216   220       +4     
=======================================
+ Hits          151   154       +3     
- Misses         65    66       +1