Add configurable CharacterSet validation regex pattern

I'm facing a situation where we need to support characters beyond the traditional "^[a-zA-Z0-9\\-\\./_]*$" regex pattern. Yeah, I can easily hard code a custom change, but it would be really nice to make it configurable. I see that this has been a pain point in the past, so I figured I would take a stab at making it configurable through the kairosdb.properties file. I tried to localize the changes to only CharacterSet.java so that there weren't too many moving pieces.

I'm totally open to feedback here! I realize this solution may not match up with everyone's vision for the code. Maybe we can hammer this into something we all agree on?

The unit tests I've run still pass, but not a few were not even passing when I forked the repo before I made code changes. I also did some manual testing with the in-memory store to verify I could send and query data. I'm sure there is more exhaustive testing that can be done, especially against Cassandra, which is our use case.