Added new_seq, a complete re-write of seq_file

Note: the contents of new_seq will replace seq_file shortly.  This will cause
      a change in the API.  At the moment testing is ongoing.

This complete rewrite came about after a discussion on biostars.org where
Heng Li pointed out how slow unbuffered zlib reading can make seq_file run
very slowly.

What we've lost:
* I've abandoned reading in a sequence a base at a time, and now only support
  getting a whole read at a time.  Modern machines can handle reading in a whole
  chromosome (~300MB) at once.

What we've gained:
* Can decide at run time to use buffered or unbuffered input and whether to use
  a gzFile or FILE to read input.
* Very fast reading (in my tests it matches Heng's kseq.h it all cases)
* Only need to add a pair of header files and link to htslib to use it
* Better format detection

What we've kept:
* Support for reading FASTA/FASTQ/plain/SAM/BAM file formats all with the same
  seamless API

I've not implemented an API for writing sequence files yet, but if there's
demand for it it should be quick to do.

Please give it a try and let me know how it can be improved. Hopefully it
provides a convenient, fast and simple project to speed work in bioinformatics.