* Fixed PowerDNS provider bug affecting endpoint conversion to
zone in a particular case
* Added a regressive test case, covered an additional case
PowerDNS provider bug: In the function ConvertEndpointsToZones,
the endpoints are sorted lexicographically increasing
(and by record type in case of match). The zones are sorted by
decreasing order of length. There's a loop through of zones
under which the code loops through endpoints. In the case
the first endpoint does not match with the first zone,
the complete zone won't get populated with any endpoints.
Commit adds:
* Implementation of PowerDNS as a provider
* Tests for said implementation
* github.com/ffledgling/pdns-go, which provides go client bindings for
PowerDNS's HTTP API, as a dependency
* "pdns" as an additional option for the `--provider` flag
* `--pdns-server` and `--pdns-api-key` as additional flags for PowerDNS
specific configuration
