bufferevent_pair: don't call downcast(NULL)
Merge remote-tracking branch 'origin/patches-2.0'
Added -Qunused-arguments for clang on macosx

The CLang provided through Xcode on Mac OSX emits warnings for
all unused include paths making the compilation extremely verbose.
Add cmake-related files to .gitignore
Merge pull request #108 from trondn/cmake-build-fix

CMake build-related fixes