thrift-as-dependency support

Hi!

First of all, thank you for your plugin! I made some major changes in it, but i'm think that you will love it:)

My proposal is to use thrift executable as usual maven dependency, because in our projects there was a problem with it, each project was shipped with binaries in source control, and that's sucks:)

To test it, you should have thrift binaries in your repo. For GAV see new constants in AbstractThriftMojo.java.

Cheers!