Add `$ pure-sass install` and `$ pure-sass -v` command line abilities.

For convenience in other projects, I've proposed `install` and `version` command line abilities.

Now, `$ pure-sass install` will grab all the pure-sass gem's sass files and place them in a `pure/` directory. `$ pure-sass -v` or `$ pure-sass --version` will print out the pure-sass gem's version. 

I've added very basic tests for the new functionality, as well. The tests introduce a fakefs development dependency, and the `install` and `version` commands introduce a thor and fileutils dependency.

TODO: Add a `$ pure-sass update` command.