fix for mobile dropdown menus + standardization of @media width amounts

Mobile dropdown menu for the 'MP3 players' category was not visible, so I included the following fix:
#menu .navbar-collapse {
	overflow: visible !important;
}
After that, the menus were still being cut off do to the -202px added to the large dropdown menu and the fact that menu with it's four columns was simply to wide to fit in the screen, so I reduced them all into a single column and made this fix:
@media (max-width: 767px) {
	#menu div.dropdown-inner > ul.list-unstyled {
		display: block;
	}
	#menu div.dropdown-menu {
		margin-left: 0 !important;
	}
}
and merged it with the other @media entry for #menu.

I also standardized the lower @media max-widths to 767 (instead of 768), because that's what the bootstrap.css file does.
Merge pull request #1002 from chimeraha/patch-1

fix for mobile dropdown menus + standardization of @media width amounts