This is a big one.

This change converts all of the existing tests from test/unit to RSpec.
It's been a long time coming, and was specifically prompted by the
inability of minitest to work with Appraisal across the various Rails
versions. Instead of ironing out that headache, converting to RSpec was
preferable (after initial probing).

Many assertions are still assertions and are housed in the Assertions
module, but they should be converted as they get touched. Similarly,
since we're touching all the tests/specs, we convert all the hashes from
1.8 syntax to 1.9 syntax.

Thanks @GMFlash!

Closes #586

(cherry picked from commit 4db98fb37e32867541f28b7832ce8d28788fd62c)

Updated documentation. Also changed the defaults of :path and :url to something more sane and defaultable.