[#1841] add IValidators, IConverters plugin interfaces
[#1841] define NameConflict error instead of using Excepion itself
[#1841] extras_unicode_convert is not a validator
[#1841] document custom converters and validators
[#1841] fix grammar, add note about __before, __after validation
[#1841] tests and fixes
Merge remote-tracking branch 'origin/master' into 1841-ivalidators-iconverters

Conflicts:
	ckan/logic/schema.py
[#1841] fix tests, pep8
[#1841] make plugins.toolkit more like a normal module
[#1841] import plugins.toolkit like a normal module
[#1841] better docstrings for IConverters, IValidators
Merge branch '1841-ivalidators-iconverters'