Add support for testing REST APIs

[jdudek]

Here's how it might look like: https://gist.github.com/1212860

We could provide default implementation of Bbq::TestClient, similar to Bbq::TestUser.

[mostlyobvious]

I used it already at least twice, so it might be good idea to have it in standard. Here's my take based on latest project: https://gist.github.com/1483073

[jdudek]

I've pushed a branch https://github.com/drugpl/bbq/tree/test_client. Code review will be appreciated.