Pileup.Element: ignore `P` CIGAR-operator

It can be ignored: http://davetang.org/wiki/tiki-index.php?page=SAM
and it is the simplest way of dealing with it correctly (if we want to
keep the padding information in the future, we will need a more complex
`Alignment` case class).
Add more tests for Pileup.Element
Pileup: add comments and rename variables
Merge pull request #55 from hammerlab/pileup_fix_and_tests

Pileup fix and tests