Compile external thrift files.

JAR files containing thrift files to compile should be added to the list
of library dependencies, but in the "thrift" configuration. The plugin will
unpack the thrift files from the JARs to an external source directory and
then compile them along with regular project thrift files.
Merge pull request #1 from strava/ls/compile_external

Compile external thrift files.
Merge pull request #1 from strava/master

Compile external thrift files.