Pysubnettree

* Configure Travis CI email recipients and build branches. (Daniel
Thayer)

* Add a .travis.yml file. (Daniel Thayer)

* Fix compiler warnings on OpenBSD, add missing include. (Daniel Thayer)

* Fix the prefixes() function to compile and work on Python 3.
(Daniel Thayer)

* Cleanup tests for the prefixes() function and improved comments
and error messages. (Daniel Thayer)

* Added prefixes() method to return all prefixes in the tree as a
set of strings, with or without length. Also supports returning
IPv4 addresses in their "native" format. (James Royalty)

* Fix a compiler warning. (John Siwek)

* s/bro-ids.org/bro.org/g. (Robin Sommer)