Removed unnecessary opening/closing of files; Optional SAM/BAM linking

0. Buffer sequences when getting fastq qual score distribution instead of opening/closing
   the file
1. Do not use file contents to infer sam/bam when opening file, instead use
   file extension only
2. Updated to latest stream_buffer.h
3. seqcat: stopped from printing emtpy sequences in 'plain' format
4. Optional _USE_SAM=1 argument to allow compiling without samtools support
5. seq_file_t->nextc not needed as ungetc_buf now guaranteed to work for at least
   one character
6. Added Heng Li's kseq.h to the benchmark folder
7. Updated README

Main upshot of these commits is we cat `cat` files together on the command line:

    facat <(cat test/*.fq)

This applies to all programs using the seq_file API

To compile without SAM/BAM support:

    make

with sam/bam support (requires htslib)

    make HTSLIB=<path/to/htslib/>
Revert num_exts to MACRO; Add -D_USESAM to README