Add library crate version of pb-rs

[UserAB1236872]

Right now this is one of the main issues prevent me from using quick-protobuf (along with #68 and a couple things in #12). It would be very useful to abstract pb-rs into its own library usable in build.rs scripts.

You can sort of do this now by installing pb-rs and executing it as a command-line tool, but it's clunky and more failure prone from a user-perspective.

[tafia]

@jackalcooper has probably done what you wanted #95.
I have not published pb-rs the lib into crates.io (I don't really know what is the best practice for having both a bin and a lib)

[tafia]

This has been done long ago now