always generate Debug symbols

In order to symbolicate crashes, we need to keep the dSYMs around.
Even or especially from Release builds. This commits enables generation
of debug symbols for all targets & configurations.
http://stackoverflow.com/a/12827463/206393 looks like a good read on
this topic.
Merge pull request #13 from diederich/alwaysGenerateDebugSymbols

always generate Debug symbols