Move .emscripten file to live within Oryol.

This moves the .emscripten file to live within the Oryol directory
structure. It is within the per-platform SDK directory so that the
same Oryol directory can be shared across multiple platforms / machines.

This prevents the Oryol emscripten build from conflicting with
other emscripten builds on the same machine.
Merge pull request #75 from waywardmonkeys/dot-emscripten

Move .emscripten file to live within Oryol.