Dev create job

So here is the meat of my work - this PR contains two commits, one which fixes the integration tests (minor change) and one which adds the createJob() method.  In order to do this, I had to add the ability to post XML because I didn't see how to post JSON (maybe if I had tried harder I could have made json work, but this was easier because I already had a ruby client that worked this way I was looking at).

I added both unit tests and integration tests to cover the new code.

Please let me know if you have any comments or changes you would like me to make so this can be merged and released.

Thanks!
-Carl

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EDIT: updated pull request taking into account your suggestions.  Also separated out the change to cause the JenkinsHttpClient to throw if a non-success response is sent, let me know if you want to discuss that decision.