Fixes some issue encountered with the official plugin repo.
1. Fixes race condition during initialisation, which would sometimes
result in intialisation failing.
2. Fixes stack traces so they reflect the C# stack trace rather than the
java stack trace (which would always be the same).
3. Adds a build script (eclipse project in the orginal repo didn't work
for me).
4. Some other minor changes (e.g. wrapping code in UNITY_ANDROID).