Fixes #193 Removes using statement around _httpClient

[ctolkien]

This change:

• Removes the using statement impacting the _httpClient - it will no longer be disposed per request
• Adds a test for this scenario
• Makes some changes with regards to the BaseAddress - you cannot change the BaseAddress once a request has been flighted. I have changed the public setter on this property. This is a breaking API change.
• Had to make some changes to the test helpers with regard to the base address.

RE: Formatting, I've tried to stick with the two-spaces format which was in use... but there may be one or two bits where it's been messed up :/

Fixes #193

[ctolkien]

Oh, it now makes MandrillApi implement IDisposable, which will dispose of the _httpClient which is reused.