Debug counter infrastructure

Reviewer: @harshsin

This module allows clients to register, unregister, and list debug counters.
The main advantage of using this module over our current approaches is that
OFStateManager will be able to enumerate the counters and send them to the
controller over OpenFlow.

Some open questions:
1. Is the 256 byte description for each debug counter overly wasteful?
2. Should the uCLI support operations on individuals or subsets of counters?
3. Should we document a naming scheme?