RFE: required HTML5 attribute support on Fields

[robmv]

Use BaseField.required_? to add the "required" HTML 5 attribute when serializing to HTML 5

Group discussion thread: https://groups.google.com/forum/?fromgroups#!topic/liftweb/Vtw0Eib8Avs

[Shadowfiend]

This should be pretty easy, but will require manually going through the fields that extend BaseField to add the required attribute as appropriate in their toForm implementations.