Use native Promise if available

[novemberborn]

Unlike say Map, the Promise shim does not fall back to the native Promise if it's available. This means we're not benefiting from unhandled rejections and other promise debugging features in modern debuggers.

Currently the shim implements the non-standard Promise#finally(). This should be moved to Task before the native Promise can be used.

[kitsonk]

I really feel we need to address this before the beta release as it has several performance implications and potentially typing implications.

A another note, related to #16 is the Promise.prototype.finally is now a TC39 Stage 2 Proposal.

My thinking is that we should do the following:

• convert dojo-shim/Promise to a "full" shim like the rest of the shims
• migrate .finally() to dojo-core/async/Task
• polyfill those features back into dojo-shim/Promise when they reach Stage 4 (which for both is likely to be at least a year)