Add support for top-level version flag

This also simplifies the check for whether a command exists.
Make top-level syntax help more accurate

The remaining manpage fixes will come later
Merge branch 'skip_pools_fix' of https://github.com/Alexqw/zfSnap into Alexqw-skip_pools_fix
Merge pull request #31 from Alexqw/populate_skip_pools

Move $skip_pools populating to function
Merge branch 'is_snapshot' of https://github.com/Alexqw/zfSnap into Alexqw-is_snapshot
Merge branch 'delete_cleanup' of https://github.com/Alexqw/zfSnap into Alexqw-delete_cleanup
Update man page with new <command> structure.
Small manpage fixes and additions