Major speedup in SamSequenceDictionary & SamReadGroupDictionary classes

Major speedup in SamSequenceDictionary & SamReadGroupDictionary classes
 * Please note that this does introduce a minor source-incompatibility,
only affecting those working directly with the provided Sam*Iterator
typedefs. The short answer is that the iterator now references a
std::pair instead of the 'plain old' data. Use the pair's "second" field
to access the desired SamSequence or SamReadGroup.
 * Doxygen docs have been updated to reflect this and provide a bit more
explanation/examples (in docs folder run 'doxygen Doxyfile' to get the
updated API pages).