Make interface more fluent

[clphillips]

Look into making the interface more fluent in 2.0.

Instead of:

->every()
->minutes(5)

Consider:

->every(5)
->minutes()

Need to look at how this impacts both every() and only(), and their respective periods. Definitely want to avoid having minutes() and minute() used for the same purpose, e.g.:

This would be bad

->every(5)
->minute()