Uncalled genotypes support

Closes issue #173 

Added support for ploidity in uncalled genotypes.  Previously "." and "./." genotypes were treated equivalently.
* Behavior of gt_nums and gt_bases attributes of _Call remains the same (both are "None" for uncalled genotypes).
* Added a "ploidity" attribute to _Call that returns 1 for "." and 2 for "./.".
* The gt_alleles attribute to _Call, which previously threw an exception for uncalled genotypes, now returns a list of "None"s with a length matching the ploidity for uncalled genotypes.