Use explicit allows in .gitignore

[adamv]

In various forks, people are adding more and more stuff to .gitignore, as those folders show up in their /usr/local location.

It might be better to explicitly ignore everything and then explicit allow the specific folders that brew uses, plus the readme.

This should eliminate the need for special-case ignores for things that just happen to sprout up in /usr/local.

[sophiebits]

Sounds good to me. I think this patch does the right thing, but don't trust me on it.

spicyj@befa6e0e4095fd2df30eea81477dcdfdc29e5a6f

[mxcl]

Great thinking.