Change AR in method and property names to Ar

[vjik]

See https://github.com/yiisoft/docs/blob/master/010-code-style.md#names

For example:
ActiveQueryInterface::getARInstance() → ActiveQueryInterface::getArInstance()
or
ActiveQueryInterface::getARInstance() → ActiveQueryInterface::getActiveRecordInstance()

[xepozz]

Either the original option or full name, not Ar