Refactor configuration to drop worker type. Refactor tests.

We only support one worker system at a time so I don't feel it's worth
having config options for each backend system. It also doesn't scale up
very well if we had further backends.

I've also refactored the tests to not use as many mocks / test doubles.
I'd prefer not to use mocks unless hitting the network as I've found it
can hide issues by testing implementation over testing effects.
Reorganize some tests to fit into the collapsed spec directory.

spec/ can be thought of as anything in sunspot/queue/.
Ignore tmp/ which stores guard output.
Guard has deprecated the version option since rspec 1 is not supported.
Bump version to 0.10.0.