add support for scrooge thrift generator

[johnynek]

See: http://twitter.github.io/scrooge/CommandLine.html

something basic can be done probably with a genrule + a macro once we solve #2.

An problem (to me) is dealing with dependencies between thrift targets. One approach is to pull the transitive thrift files, compile all of them using --gen-file-map, then only put the output classes from the current target into the current jar. This may be too slow for deep dependencies however.

[johnynek]

see: https://github.com/wt/bazel_thrift