MetaCache 2.5.1

Minor fixes & improvements:

• fixed/updated the documentation and clarified that input files can be compressed if MetaCache was compiled with zlib
• the MetaCache executable returns EXIT_FAILURE (usually 1) if any fatal error occurs that causes the program to abort

Improvements to utility scripts (thanks to @Gullumluvl):

• the metacache-build-refseq script should work from any directory now
• sped up the final unpacking step in download-ncbi-genomes script