engine entrypoint plugins

[Mike Bayer]

the 1.0.3-added dialect "extra entrypoint" system is both too heavy handed and also not very functional. The attached patch illustrates a new system that provides for individual plugins specified as query string parameters, staying away from the driver portion of the URL that systems like Openstack rely heavily on. It also provides stateful hooks so that the URL format and kwargs can both be fully extracted and applied to the engine after its constructed, fixing the awkwardness of the dialect system that calls upon class-level hooks.

The 1.0.3 system is just two method calls which are of little use compared to this newer system, they should be marked deprecated.

tagging this at 1.1 but it may be very useful to have this in 1.0.9.

[Mike Bayer]

• Added a new entrypoint system to the engine to allow "plugins" to be stated in the query string for a URL. Custom plugins can be written which will be given the chance up front to alter and/or consume the engine's URL and keyword arguments, and then at engine create time will be given the engine itself to allow additional modifications or event registration. Plugins are written as a subclass of :class:.CreateEnginePlugin; see that class for details. fixes #3536

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