Fixes #13

I added an error to raise when someone tries to use an unconfigured client.

I also refactored tests a bit - moved error testing to its own spec file, etc. One important consequence of https://github.com/PeterBorah/vigor/commit/5d49f785d236887b0a659c5323fb4276a4ad3f21 is that Vigor is effectively a singleton now. That means that all tests are referring to the same Vigor object, so configuration carries over from test to test - not ideal for testing. I set a before hook in ``spec_helper.rb`` to try to isolate tests better:

```
  c.before(:each) do
    Vigor::Client.default_options = {}
  end
```