Setters for the simple members of the Sam API

only those whose data can be accessed independently inside the bam1_t
and bam_core_t structs. Basically the ones with fixed length.

* add simple members for alignment fields
* add simple flags
* add tests to all functions

this closes #13
Merge pull request #19 from MauricioCarneiro/mc_simple_setter_api

Setters for the simple members of the Sam API