Add nonce_type support

Allows you to specify that you'd like to use the 'tonce' parameter
rather than 'nonce'.  This is useful for when you have multiple
threads/processes communicating and you can't be sure that a given
request will block before another occurs, etc.

It doesn't change the default behaviour, but you can configure
`client.nonce_type = :tonce` and it will change the parameter sent.

The test added with this to test that the request includes tonce rather
than nonce is not valuable until
https://github.com/bblimke/webmock/issues/297 is fixed.  It's still the
test that should go along with this.
Merge pull request #48 from knewter/feature/allow_tonce

Add nonce_type support