3.9

[titusfortner]

Ok, this is everything I think we need as a final 3.x release.
This might be a pain to review, but I tried to make each commit obvious and straightforward.

This is all dependent on the refactoring in #86, #90 & #91
Caching addresses #29 & #77
Direct downloading of geckodriver addresses #30

I've got cache time defaulting to zero, with the idea of making it default for 4.x. Or should we set a value for 3.9?

I'm thinking we release this and then for 4.0 we:

1. Add the documentation
2. Create the Rake Tasks
3. Remove the deprecations
4. Change default cache_time to 24 hours

[titusfortner]

Only thing left to evaluate is #104 so closing this.