Closes #13
This should be the fix for #12. However the issue will stay open until tested with Xcode 8.3.1
Both CSSCodePoints.h and HTMLTokenizerCharacters.h define the same symbols. They are never included by the same compilation unit, but when they are compiled into the same module, they create a conflict. Fix this conflict by using `textual header`.
Move the `module.modulemap` into `Sources/include` and add an exclude for `Tests/Fixtures`.