Various small fixes

Fixes include:
The addition of a jshintrc. Main.js was adjusted so the it can be successfully linted. (See also https://github.com/komola/paymill-node/issues/9)
For better error support the API calls should throw a TypeError instead of calling the (given) callback with a string. (See last comment in issue https://github.com/komola/paymill-node/issues/7).
Example 7 had a wrong file extension.
Add the error event in setup_response_handler.
Setup_response_handler function does not need to be called if no callback was given.

There are a number of changes (and some of them are rather opinionated) and I do not know if you are really interested in pulling it but perhaps some of the changes could be useful for you.

Regards
nponiros