SamBuilder, in-place setters, partial Sam tag support

-Created SamBuilder to build Sam objects from existing data or
 from scratch
    -unlike in-place setters in Sam class, can restructure and
     manipulate the variable-length read data in arbitrary ways
    -offers both repeatable builds and the option of a (slightly
     more efficient) one-time build
    -supports setting data fields using existing reads, vectors,
     initializer lists, and strings (least efficient)
    -basic validation of logical consistency of read before each
     build (eg., number of base qualities must match number of bases)

-In-place setters for data fields in the Sam class when more radical
 restructuring is not needed

-Basic support for Sam TAG:TYPE:VALUE fields
    -read-only support for integer, double/float, char, and string
     valued tags
    -can copy an existing read's tag in their entirety using SamBuilder,
     but can't yet set individual tags

-Cigar::make_cigar_element() to ease creation of vectors/initializer_lists
 of cigar elements

-Comprehensive tests for SamBuilder and SamTag

TODOS:

   -support for tag fields of type H (byte array) and B (numeric array)
   -add ability to set/modify individual tags in SamBuilder
   -properly set the core.isize field in SamBuilder (currently the
    only field not yet fully supported apart from the tags)
   -fix issue with htslib bam_aux2i() and bam_aux2f() return value
    ambiguity when field is of wrong type
why not follow the empty() == true convention of the STL and all our other objects?