Add override directive to makefile

Add override directive to avoid flags overridden by custom settings.

Then make CFLAGS="$(XXX)" will honor the existing flags in the makefile defined by upstream.
Add manpage rules.

Add rules to install manpage.
Merge pull request #29 from cicku/patch-1

Merged. Thanks !
Added a debug CLI flag, disabled by default, so cv is now quieter.
Added a small tip about ncurses.
Updating to 0.5 (less verbose)
Debug mode was broken. Oops.
Merge remote-tracking branch 'Xfennec/master'