manager checks: tests + implementation + doc
added release note about deprecating DatabaseValidation.validate_field
changed mysql field validation -- replaced hardcoded set of fields with checking field.db_type()
introduced checks.{register, run_checks} functions

- added warnings support: validate doesn't raise CommandError when there
  are only warnings, introduced color support
- replaced get_validation_errors calls with run_checks
doc -- added section about registering checks
deleted hints from custom user model errors
removed more hints
renamed BaseCheckError -> BaseCheckMessage
started rewriting error formatting
improved BaseDatabaseValidation.validate_field doc
small clean up (deleted unused imports)
removed string formatting with locals()
added some docs to Model._check_* methods
cleanup: renamed d -> context
changed custom user model checks -- raise Warning instead of an Error
changed formatting errors
split django/core/checks/base.py into smaller files
renamed: checking framework -> [system] check framework
fixup in ManyToManyField._check_relationship_model -- checking missing through model
added skipping autocreated through models