Add DialURL and ModifyRequest.Increment

add modify-increment extension support detailed in RFC4525 (#227)

* add modify-increment extension support detailed in RFC4525

This extension is detailed in [RFC4525](https://tools.ietf.org/html/rfc4525).
To summarize, it allows for auto-incrementing a specific attribute by a
specific amount. It can be used in conjunction with pre-read, post-read,
and assert control types

* update v3