add support execute mb from global node modules

[winterAndvelvet]

Hi all, when i use mbtest,i install mountebank using npm install -g mountebank ,but it is not work.I found in mbtest sorce code,mountebank must be installed in a path same level with execute mbtest.I think the path should not be hard code.so,i try to fix it.

Regards, Winter Wang

[brunns]

Thanks. Looks good on the whole, but there are a couple of build failures. If you run make precommit locally, you can see and fix these issues.

[brunns]

Does #55 supercede this?

[brunns]

Assuming #55 superseded this issue. Please reopen if that's not the case.