Improvements to Ctrl+C handling

Changes:

• When mbedhtrun catches Ctrl+C it will return with error code < 0. See: ARMmbed/htrun#73.
• Greentea will check mbedhtrun return code and stop execution test cases with mbedhtrun return to environment value < 0. For example -3 stands for KeyboardInterrupt. See htrun/v0.2.4.

Released to PyPI:

Submitting dist\mbed-greentea-0.2.6.zip to https://pypi.python.org/pypi
Server response (200): OK