Modified `debian/nsnake.desktop` according to suggestions from @jnumm

Don't need to specify full file path on the desktop file's "Exec" field,
as long as it's located on $PATH
Removed .xpm entries on  `debian/source/include-binaries`, thanks to @LocutusOfBorg

xpm files are not binaries, no need to include them here.
Updated Vcs-Git link on `debian/control`, according to suggestion from @jnumm

Debian Policy Manual section 5.6.26 specifies that Vcs-Git fields consists
of a repository URL and (optionally) -b with the branch name of that
repository.
Bumped standards version to 3.9.5 on `debian/control`, thanks to @LocutusOfBorg
Removed extra tab on `debian/rules`, thanks to @jnumm
Added a Brazillian Portuguese GenericName to the desktop file