Allow installation via melpa

It works by bootstrapping from the melpa installed code. The alternative would be to have el-get sometimes be a `:type elpa` recipe and sometimes be a `:type github` recipe which I decided would be too complicated.

I've tested this with the melpa sandbox target, but of course the real test can only come after merging when it's picked by the melpa server.

I'll close some installation failure issues that haven't seen any progress; this doesn't exactly fix them, but at least offers another option.

close #1771
close #1680
close #1245
close #1128