Sync with docs

[erik-perri]

This updates the various rules so that their description is back in line with the Laravel docs site. In addition to the docs changes, some rules had either been updated since implemented or had functionality missed in their implementation:

• timezone supports options
• requiredUnless handles the null case
• dateFormat supports many formats
• alpha[Dash/Digits] support an ascii only option
• before and after date rules support a related field name instead of a date