Remove 'old' canvas plugin.
Unify include to support js and jse.

* include is now async for JSE files.
* include now works for arbitrary urls through a JSONP proxy.
* include and execute are now a keywords.
* includeJS keyword removed.
* Generated JS may yield for includes, now executes a callback to signal termination.
* executeFile* functions have now been removed.
* include_js function has been removed.
Merge pull request #27 from EMGroup/unified-include

Unified include