Allow for setting individual options

Currently, the `Glicko2` class expects a complete `settings` _Object_. Thus, if
one wanted to only change, say, the default rating, they **cannot** simply do
this:

  var glicko = new glicko2.Glicko2({
    rating: 1600
  });

Instead, one must pass the entire `settings` _Object_, even if they want to
change just one value:

  var glicko = new glicko2.Glicko2({
    tau: 0.5,
    rating: 1600, // but I only wanted to worry about this :(
    rd: 350,
    vol: 0.06,
    volatility_algorithm: 'newprocedure'
  });

This patch fixes this issue by checking for each individual setting in the
`settings` _Object_ instead of just checking for the entire _Object_ itself.
This way, one does not have to pass a complete `settings` _Object_ if they only
want to change a few values.

An alternative way to do this would be to _extend_ the `default_settings`
_Object_ (which this patch removes) with whatever is passed as `settings`.
npm modules like xtend and lodash.assign provide this functionality, but in
order to maintain Bower support (which doesn't support `require()` calls), I
would have had to setup a whole build system that would inline one of these
modules before publish. Way too much work, especially for a tool I do not use.

Closes #5
Merge pull request #5 from KenanY/individual-settings

Allow for setting individual options