Extracted name resolution
Cache address lookups

This prevents a delay in processing if a name for the address does not
exist.  Now the address is checked only once, at the cost of storing
each lookup.

Since the same object is returned from the cache, special care is taken
to always return a new name.
Fixed gem installation

There is a mismatch between rake-extensiontask and extconf.rb with
respect to the destination of the generated shared object.

Also, the extension was not included in the package.
Install libpcap for travis-ci builds