Introduce caching to circumvent issue with using UserAgentParser.parse

[hexgnu]

What

So it seems to be a pretty recurring issue based on previous PR's that UserAgentParser.parse can cause performance issues. I found this out the hard way by loading 30 million user agent strings and not realizing that was how it was defined.

This PR aims to help by introducing a cache that allows users to still rely on the same functionality but enjoy the much faster speed.

How to test

I threw a few cache specific tests in there but also made sure the tests themselves still worked.

[toolmantim]

You don't think the two mentions in the Readme is sufficient?

and:

[hexgnu]

Yea I realized very quickly and moved over to loading a constant.

I just personally felt that UserAgentParser.parse always loading the yml from the file system was unneeded since you could have effectively the same API with better performance.

[toolmantim]

Yeah, I understand. It's just implementing our own instance cache feels like a problem that shouldn't be solved in a gem that parses user agent strings. And I'm not sure of the side-effects of adding a cache.

Perhaps we could just ditch the class method?

[hexgnu]

I am 100% for ditching the class method.

[toolmantim]

Cool, I've opened #14 to remove the class method on next major revision bump.