Make BaseUri Overridable

[eaglestorm]

Hi currently the MessageMedia Base Uri cannot be modified as it is hard coded, so we are unable setup integration tests.

Can this be modified so we can setup a mocked server for integration testing.

[AaronCoad]

Not sure if it helps, but we ended up downloading the source code and adding it to our project in order to resolve our issue with the status enums.

Doesn't look like the SDK is really being maintained.