Support for mips/mipsel

Fix for issue #98

Avoid unaligned memory access on architectures that don't support it.
Fixed endianness related issues. 
Further fixes in https://github.com/jkbonfield/htslib/tree/SPARC

I have now tested that branch with both make check (htslib only) and also ported sam, bam and cram in both directions between sparc and intel and verified they are compatible. (I didn't test BCF, sorry.)

This seems to be a more complete test that the one provided in this pull request, but I'm not sure what to do with it. Ie do I do my own pull request? If so to whom? Hence I'm passing the buck and will let someone just look over this branch and merge in :-)

James

PS. The SPARC uncovered lots and lots of unaligned memory accesses. So many that I can only assume the MIPS system does indeed permit these without throwing up an error, albeit possibly not in an efficient manner.  Hopefully my ua_* functions will compile down to the same code they originally replaced on x86 systems.