Proxy Support: removed

Ok, I hope this will not infuriate you :-)
I removed my changes that were adding proxy support in favor of a more nodeJS-like approach.
I have added an optional constructor parameter to the AlexaAPI function, called 'opts'.
Whenever a client is created with this extra parameter, the main URLRequest object is wrapped for that particular client using the request.defaults() method (documentation here: https://github.com/mikeal/request#requestdefaultsoptions ).
This ensures that 2 different alexaAPI clients could use 2 different sets of HTTP options.

I think this is a more flexible solution, that empowers the clients of this API to alter the underlying HTTP behavior if they wish, and at the same time it does not break compatibility with the existing code.