Fix Compile errors reported in #16

Public extensions of generic classes are apparently not supported
in the version of Swift in XCode 6.1. So for now I've made them
internal so others can at least compile.

I can't run the tests because of #17. I'll submit another pull
request with a new test runner.
Merge pull request #18 from michaelgwelch/fixExtensionsCompileError

Fix Compile errors reported in #16