pxpcap: Add lock when compiling filters

Looks like the filter compiling function isn't threadsafe, so this just
adds a lock around it.

Thanks to @colin-scott for raising the question of thread safety in pxpcap.
libopenflow: Update _wire_wildcards docstring

In particular, this points out that the issue is relevant for OpenFlow
1.0 as well (made clear in 1.0.1).
util: Make hexdump() work for bytes objects too
util: Add/improve docstrings and comments

This adds some docstrings, makes others more informative, and adjusts
some to comply with POX convention.  Also adds some notes/TODOs.
util: Add DefaultDict class

This is a subclass of defaultdict which passes the missing key to the
factory function.