Integrated igzip library to enable fast compression of BAM files.

igzip enables faster level 1 compression on BAM files compared to zlib while providing similar compression ratio as zlib. (https://software.intel.com/en-us/articles/igzip-a-high-performance-deflate-compressor-with-optimizations-for-genomic-data). 
Given that these two intrinsics are unlikely to be present on non-intel platform they need some kind of #ifdef guard.