Make interface more fluent

Look into making the interface more fluent in 2.0.

Instead of:

``` php
->every()
->minutes(5)
```

Consider:

``` php
->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**

``` php
->every(5)
->minute()
```
