support out-of-source build
remove custom ExportHeader function, use install instead
* importing PBGZF
* bamtools merge now uses mutiple threads for writing (not tested)
* constructor for pbgzf was not setting the correct number of threads
* minimum # of threads is 1
* adding missing files
* adding -pthread compiler flag
remove -pthread from global CMAKE_CXX_FLAGS_RELEASE settings, add -pthread to BamTools target
fix zs.avail_out field
set SOVERSION to 2
Fixed: convert tool - improper unsigned char handling in tags
Merge remote-tracking branch 'upstream/master'

Conflicts:
	src/api/CMakeLists.txt
	src/toolkit/bamtools_convert.cpp
	src/toolkit/bamtools_merge.cpp
	src/utils/CMakeLists.txt
Merge remote-tracking branch 'upstream/master'
fixed compiler error: BamAlignment and SamHeader is a struct, not a class
bugfix: if the number of elements of the tag vector data (e.g. FZ flow intensities) is a multiple of 256, e.g. 256, 512, 1024, etc, the number of elements saved to the bam file becomes 0, try to replace strcat with memcpy
Merge bamtools v2.3.0 '9cfa70bfe9cdf1b6adc06beb88246b45fdd6250a'
removed inline keyword to fix build with clang
Merge branch 'master', remote-tracking branch 'upstream/master'
comment out unused variable elementLength