Allow setting all request options

[eladnava]

As discussed in #179, we'd like developers to be able to set all request options externally, by providing an object to the gcm.Sender() constructor, such as the request proxy, timeout, strictSSL, etc.

Changes:

• Require lodash
• Removed manual checks for proxy, maxSockets, timeout, and strictSSL request options
• Added a variable sender_options that contains the request options passed to gcm.Sender()
• Added a variable request_options which uses Lodash's _.defaults() function to prevent overriding our internal options with externally-provided ones
• Allow overriding timeout externally if provided, if not, use Constants.SOCKET_TIMEOUT (same as old behavior)
• Modified post_req to use the request_options instead of the old post_options which was manually constructed

Checked and works. Ran npm test successfully. External options do not override (except timeout and new headers)

Closes #179.

[hypesystem]

Just beautiful! Great job :) --- had one comment

[eladnava]

@hypesystem Fixed. Thanks for the comment! :)