Add ExtendedDismaxParser support

[diebels727]

Now that Solr 3.5 is in the mix, I independently implemented support for the edismax parser, which is eerily similar to the implementation described in Issue #93.

I still have a failing test regarding the dynamic_fields shared examples; I believe this is a result of some of the wizardry I am attempting to do reuse the shared examples in the tests for edismax.

[njakobsen]

Closed by #487 because Sunspot now uses edismax. We incorporated your edismax tests in #488. We took the easy route this time, but we'll see if there's a reason to support both in the future, at which point we may revisit this pull request. Thanks for the help!