scrooge: Include (and optionally compile) external thrift files.

RB_ID=154409
Replace backslash with forward slash in file URI

Signed-off-by: Jeff Smick <jeff@twitter.com>

RB_ID=154844
fix bug:thrift idl containing UTF-8 produces java.nio.charset.UnmappableCharacterException

Signed-off-by: Jeff Smick <jeff@twitter.com>

RB_ID=154840
update changelog
bump version to 3.1.8