Add filters to preload and presave

This makes it easier to extend without declaring stub classes
allow the Fieldmanager_Field:: to accept non key-value attributes like 'required'
was throwing a warning when 0 was not a valid offset
Merge branch 'master' of git://github.com/alleyinteractive/wordpress-fieldmanager into allow-non-keyvalue-attrs

Conflicts:
	php/class-fieldmanager-field.php
allow standalone attributes to be passed as boolean true
unit test for attributes
use regexp to test for attribute surrounded by whitespace instead of actual space char
attributes can be followed by whitespace or slash
Merge pull request #230 from alleyinteractive/fix-warning

check isset before is_array
Merge pull request #223 from alleyinteractive/allow-non-keyvalue-attrs

allow Fieldmanager_Field::attributes to accept non key-value attributes like required
Merge pull request #191 from alleyinteractive/filters

Add filters to preload and presave