Refactor purger to use Net::HTTP

We can do this by adding custom verbs to the Net::HTTP engine, allowing
us to make PURGE and DOMAINPURGE requests.
Unit tests for Purger

These use WebMock to fake HTTP responses.
Unit tests for Spider
Add Travis CI config file
Unit tests for PagePurger
Unit tests for DomainPurger
Add Travis status to README
Merge pull request #12 from robmiller/unit-tests

Unit tests and CI