Make sure last argument of CGBitmapContextCreate has type CGBitmapInfo (resolves #23)

Changes the type of the last argument to a call to CGBitmapContextCreate
(without changing the actual value passed in the argument) to silence
a compiler warning in Xcode 5 DP.
Merge pull request #24 from inkling/aegolden/misc_fixes

Make sure last argument of CGBitmapContextCreate has type CGBitmapInfo (...