Auto download/update layers specified in g:layers

[cweagans]

We shouldn't need to download layers separately to set up vigor. Maybe something to this effect:

	" Given a layer name, download or update the local copy.
	function! DownloadUpdateLayer(name)
		if !isdirectory(g:vigor_home . "/layers/" . a:name)
			if a:name =~ "/"
				" If the name has a slash, then we'll download the Github repo specified.
				execute "silent !git clone https://github.com/" . a:name . ".git " . g:vigor_home . "/layers/" . a:name
			else
				" Otherwise, we're going to treat it like an official layer in the vim-vigor namespace.
				execute "silent !git clone https://github.com/vim-vigor/" . a:name . ".git " . g:vigor_home . "/layers/" . a:name
			endif
		else
			if !exists("no_vigor_autoupdate")
				execute "silent !git pull -C " . g:vigor_home . "/layers/" . a:name
			endif
		endif
	endfunction

And then iterate over g:layers in the bootstrap function. Not sure what implications this has for plugin installation.