[#1841] add IValidators, IConverters plugin interfaces
[#1841] define NameConflict error instead of using Excepion itself
[#1841] extras_unicode_convert is not a validator
[#1882] Fix org link in dataset page

Replaced use of `with` in the snippet.
[#1841] document custom converters and validators
[#1883] bump solr schema version to 2.1
[#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
rewrite tracking summary update query
[#1883] use schema version 2.3 instead to match ckan version
[#1883] apply 2.3 schema change to multilingual extension as well
Merge branch 'wardi-1883-bump-solr-schema-version'
Update related.py
Update test_related.py
Merge branch '1841-ivalidators-iconverters'