function to create a refspec to fetch
fetch specific refspecs

This is in preparation for fetching pull request branches.
make failed builds red (fixes #22)
DRY up running build steps
add build steps for setting GitHub status
add STARTING message to build output
simplify running a process (avoids race condition)
better build error handling (fixes #21)
fix tests I broke :hushed:
:lipstick: pretty, terse code
add pull request event request model
add request handler for pull request events
update README for pr branches
Merge pull request #23 from half-ogre/half-ogre/4-fork-pr-branches

Build PR branches