Skip Weaver if an appropriate python is not found.
(Avoids a complete build failure.)
Merge pull request #600 from dthain/master

Skip weaver if python not found.
whitespace
Return bytes added to buffer.
Replace "../src/chirp" with function call.

This change simplifies the Chirp tests, prints out the chirp command run for
debugging, and allows us add/change arguments to the command (e.g. add debug
flags).