Enable some more aggressive compiler errors
Warn about unreachable code
Always enable assertions
Add executable_path/Frameworks to rpath

I don't fully understand the implications of this change, I just know
that it fixes errors of the form:

dyld: Library not loaded: @rpath/libswift_stdlib_core.dylib
Merge pull request #28 from jspahrsummers/more-xcode6

More Xcode 6 stuff