Add config singleton

As suggested in issue #15, this PR adds a config singleton. 

I decided to implement this feature before #15 as doing #15 would further complicate the config options.

Notes:
* Because of how node.js handles modules, I was able to implement the config library as a singleton without needing a `getInstance` function as was suggested in #15
* I've decoupled command line parameters and config values . E.g. the `--bucket` cli param is now stored in the config as `s3Bucket`. This should allow flexibility (re)naming config variables without having to change the cli interface.
* As a result of the new config library, I was able to simplify the parameters for a few constructors. This does however break backwards compatibility again.

Also, in order to prevent larger diffs down the road, I've attempted to infer what the coding standard is for this project, and have reformatted a few things as a result:
  * Added docblocks to all functions and classes
  * Changed underscore variable names to camel case
  * Converted spaces to tabs

As a result, there are a number of whitespace changes, and it's best to view the diff for this PR with [whitespace ignored](https://github.com/bcoe/thumbd/pull/18/files?w=0)