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 know performance isn't important here, but you could save these copies by creating a `ReadGroup` constructor that takes in two pointers of characters *note this is not a c-style string*, and iterates from start to end parsing it.