Evaluate OS X support

[robo9k]

Right now no effort is made to build this crate on OS X.

One would need to evaluate the requirements (building/downloading/installing libmagic) and possibilities for continous integration.

[robo9k]

Travis CI seems to be able to build on Mac OS X (but support is still considered beta, not sure if Rust works at all).

[anna-is-cute]

Howdy! I develop on OS X, and after installing libmagic via homebrew, this crate works as expected. I currently use it in bins, and it works completely fine.

Basically, to use this crate on OS X, one just has to run brew install libmagic, and that's it.

Not sure if this is useful feedback or not, but just figured I'd give an update.

[robo9k]

That is quite useful feedback, thanks @jkcclemens !
I have updated the README accordingly and intend to add OS X to the Travis CI build soon™.

I'll leave this issue open in case I need or find more information 😃