Add directory of all registered thrift files to the include path

This allows writing thrift specs that reference specs in other
projects.
When requiring, add all thrifty build directories to $:

This ensures that cross-directory includes work correctly.
Exit in a slightly less lurky way
Add a test that including thrift files in different directories works

Because the thrift compiler seems to be rather insensitive to reserved
tokens occurring even in contexts where they should be identifiers,
rename "service.thrift" to "user_service.thrift".
Merge pull request #1 from ebroder/includes

Add directory of all registered thrift files to the include path