Library template

Adds a [library application](http://learnyousomeerlang.com/building-otp-applications#library-applications) template and command-line tools:
* shell completion
* `rebar -c` help
* `rebar create-lib libid=…`

A library application is the Erlang/OTP way of shipping commonly-known libraries.

It is simply an application stripped-down form its `*_app.erl` and `*_sup.erl` modules and which features a slightly different `*.app.src` file.