Added support for field options to FieldBuilder

Currently, there is no way of cleanly adding options to mapped fields using the FieldBuilder. This makes it impossible to provide the "unsigned" option using the fluid API.

The current solution is to provide options directly to ClassMetadataInfo::mapField() but, for consistency in my project, I'd like to use the FieldBuilder.

This patch adds two new methods allowing arbitrary options to be mapped to a field, and a shorthand method for enabling the "unsigned" option.

Unit test included.