Add Content-type headers for requests
Fix a mistaken __getattr__ later when this variable is looked for
ntlm: raise HTTPErrors when appropriate. Fixes issue #3

It turns out that the ntlm auth handler provided by python-ntlm
doesn't handle http error codes. To work around this I created a
subclass of HTTPNtlmAuthHandler that calls the 'error' method of
it's parent if a non-200 status code is returned.