Incorporate bamtools' own and my custom changes
Bamtools incorporated methods for returning a reference
to a const copy of the BAM header information, so use
their version rather than mine, I was already doing that
but needed to move it out of the _BAMTOOLS_EXTENSION
define but did surround it with _IF_BAMTOOLS_BROKEN if
something were to go wrong with it.  I also made sure my
other bamtools extensions (Find() and ConstFind() methods
for SamSequenceDictionary) were within a
defined(_BAMTOOLS_EXTENSION) && !defined(_IF_BAMTOOLS_BROKEN)
so we can revert if necessary.  I submitted these last
changes as a pull request to pezmaster31/bamtools.