Docs: add a user guide.

Having only API docs sucks.  Let's not suck.
Docs: add contributor guide.
Docs: Add instructions on additional download method.

As Danny pointed out, there is another use-case I didn't address: people
who have git on their systems but aren't using it for whatever project
in which they're embedding Alertinator.
Docs: Clarify the purpose of our customizability
Docs: Clarify how alert methods work.
Merge improve-documentation (#1) into master

Docs: add user and contributor guides.

Because a lack of documentation is a lack of respect for your users.