configurable zip compression

Have zip format honor compression-level.
Merge branch 'patch-1' of https://github.com/vlovich/libarchive into vlovich-patch-1

Conflicts:
	libarchive/archive_write_set_format_zip.c

This was originally submitted as:
   https://github.com/libarchive/libarchive/pull/84

I've made the following corrections:
   * Added a new variable for the deflate compression level (the submitted patch mis-used the requested_compression variable)
   * Extended the Zip store format test to also verify correct results with compression-level=0

TODO: This really needs a new test to exercises non-zero compression-level options.
Merge branch 'vlovich-patch-1'