Added tests, simplified syntax

This update most importantly adds some tests.

But it also replaced `timeSpecifier.timeAhead` with `timeSpecifier.ahead` and `timeSpecifier.timeAgo` with `timeSpecifier.ago`, as `timeSpecifier` gives the user enough context to infer what `.ago` and `.ahead` mean. This also allows us to use the much cleaner syntax `3.days(.ago)`.

This also overloads the functions such that `3.days()` returns an NSTimeInterval representing 3 days.