(#80) - Use nomnom for argument parsing

Why the switch
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1) optimist [has been deprecated](https://github.com/substack/node-optimist)
2) As part of #72 I'm likely to add new options soon (ones for cors e.g.) Having usage info in 2 places instead of three is handy then.
3) As part of #72 this project will become more complicated. Things like binding to a new port/ip while running need to be supported, as well as changing the PouchDB object while running. Less global variables and validation of these variables later on will definitely help then. Nomnom helps in both cases.
Seems like https://github.com/jshint/jshint/issues/1313 prevents me from doing that (my best guess is that it sees the node option as being inside an anonymous function). Wrapping the argument parsing inside a function and disabling maxlen for that function only works, so I used that as a workaround.