Add support for AD8495/6/7/8

Tested for AD8495 using circuit from their datasheet and appnote.
http://www.analog.com/static/imported-files/data_sheets/AD8494_8495_8496_8497.pdf
The AD849x devices can drive up to 1k load so standard 4.7k pullup that may already be present on existing boards should not cause problems.