Minor GeoIP doc improvements (Refs LOGSTASH-1703)

Escaped markdown, fixed "its" typo, improved capitalization and
added bettermap link--in hopes there will eventually be a README
there.
Escape markdown in filter base class comments
Merge pull request #883 from bfritz/LOGSTASH-1703

 Minor GeoIP doc improvements (Refs LOGSTASH-1703)

Thanks for making Logstash more awesome!