Refactor code to be more Nodejs-like

[larsgw]

Currently, all the source code (minus dependencies) is in one file. This is because, previously, that one file was used in the browser as well. Now that we have a browserify bundle (see #24), this isn't necessary anymore, and code should be split up into different files.

Also, I followed an... ehmm... unusual coding convention when I (re)wrote the source code in v0.2. I'll fix that too.

Some other expected improvements:

• Use prototype instead of reassigning methods on each Cite construction [EDIT: done (Use prototypes and expose version number #33)]
• Make version accessible from the Cite object [EDIT: done (Use prototypes and expose version number #33)]
• Deprecate log-related methods
• Make Cite#data read-only [EDIT: this can't actually be done]
• Probably some more things

[larsgw]

Edit: The following aspect is now handled by #38

This probably includes using more recent ECMAScript features like const, let, arrow functions, template strings, Array#map() (and other array and object methods) etc. These can be parsed for browsers with https://github.com/babel/babelify (or maybe something similar).