This commit changes the 'FromForm' trait in two ways:
1. The singular method is now named 'from_form'.
2. The method takes a second parameter: 'strict: bool'.
The 'strict' parameter is used to specify whether form parsing should
be strict or not (i.e. lenient). When parsing is lenient, extra form
fields do not result in an error. This lenient behavior is used by a
new 'LenientForm' data guard type to request lenient form parsing. The
behavior for 'Form' remains unchanged.
Resolves#242.
Sergio Benitez