ReadGroup class along with some support in SamHeader.

ReadGroup is a simple class for storing a read group (@RG) record from a SAM header.  This commit also adds a method in SamHeader that parses the header text and gets a vector of ReadGroups.

I tested this within foghorn and it works fine.
I'll trust you that this works, but I would expect the read group string to be terminated by `0`, isn't it? I guess the tests will tell us what's the truth.