Add API control of the signal disposition.

See http://reviews.llvm.org/D4221 for details.

This commit allows you to control the signals that lldb will suppress, stop or forward using the Python and C++ APIs.

Change by Russell Harmon.

Xcode build system changes (and any mistakes) by Todd Fiala.  Tested on MacOSX 10.9.3 and Xcode 6 beta.  (Xcode 5 is hitting the dependency checker crasher on all my systems).