skeleton l2table module
l2table implementation

The l2table datastructure maps MAC/VLAN keys to an output port number and 32
bits of metadata.

The datastructure is designed to be cache-efficient, using an open-addressed
hash table instead of separate chaining.
l2table benchmark

Inserts a large number of entries into a l2table and measures how long it takes
to look them up.
Merge into master from pull request #3:
specialized layer-2 hashtable implementation (https://github.com/floodlight/ivs/pull/3)