Make the $alias parameter in the `from` method optional

~~The refactoring commits can be merged first using PR #645~~

The approach here is the one briefly discussed at #590 and originally suggested by @guilhermeblanco
Err? What? Did you misread the diff, or are you talking about `$knownAliases` and `$fromClauses[$tableReference]`? I'd find it very odd to align those. Then again, I find aligning in general odd.