Change API to use a default engine

[dlongley]

The API for using this library should present the solve/verify functions at the top-level via a default engine. An engines property can be exposed to allow other engines to be selectively used. Switching out the default engine at any point in the future should have no effect other than performance.

[dlongley]

Also, the verify API should be async (return its value via a callback).

[davidlehn]

Also consider security concerns of loading the API via a string:
7dd8d17#r32812169