Adding support for array and hash params

[roberthoner]

See issue #11.

params do
  Array :array, type: Float, within: (0..1)

  Hash :hash, strict: false do
    # Another params block!
  end
end

There is an interface breaking change here, so we may want to put this in 0.5.0. nil is no longer encoded as a string "null", it's now encoded as a param without a value (e.g., a=value&b&c=value, here b would be nil).

This will cause a problem for any client that needs to be able to send nil up to a service.

[kayvonghaffari]

LGTM. Reviewed w/ @roberthoner and all the tests pass