Variant: make qual float and handle missing values

The missing values in htslib are represented as special bit patterns and
the user is given some macros to test if the result they just got is a
missing value or not. While I am not sure that this is the best API for
Gamgee, I replicated that for float just so we can have some
functionality to work with. The final implementation for all other types
(hence the other types of int) is up for debate.

closes #99 
Of these options, I think I like the templated universal gamgee::is_missing() approach the best, assuming we could make it work transparently with all types. 

Having wrapper objects around everything (including primitives like quality scores) with an is_missing() method (like we did for Sam tags) is another option, but would probably introduce way too much overhead.