Support for searching in gzipped files

I've added new functionality will search the contents of gzipped files. It has some initial infrastructure in place for other compressed files (ZIP, UNIX compress), but these aren't implemented yet (looks like miniz http://code.google.com/p/miniz might actually be useful here).

This change adds a new command line argument (-z, --search-zip). Searching in zipped files is off by default currently.

This change adds a new dependency on zlib. I've updated configure.ac to reflect this.

Most of the new code is in decompress.c, but I did write a small little wrapper around search_buf in search.c to check for whether the buffer contains compressed data.

I hope this is useful -- it certainly is to me!