Closes homebrew/linuxbrew#16

Closes Homebrew/homebrew#20903.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

brew-cleanup-installed: uninstall all non-whitelisted Homebrew formulae.

Useful for maintainers/testers who regularly install lots of formulae
they don't actually use.

Populate ~/.brew-cleanup-installed with the formulae you want to keep
installed. All others will be uninstalled when brew-cleanup-installed is run.

Closes Homebrew/homebrew#18300.
Closes Homebrew/homebrew#20891.

Closes Homebrew/homebrew#10817.

Closes Homebrew/homebrew#20813.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

We previously tried to call llvm-g++ under the name `g++`,
but this is now a link to clang on Xcode 5+.

Fixes Homebrew/homebrew#20691.

gunzip can only extract files in-place, so just shelling out to
gunzip was actually leaving the uncompressed file in Homebrew's cache,
not in the temporary directory. (It also destroyed the original
compressed file.)

References Homebrew/homebrew#20716.

The implementation of #eql? and #hash should ensure that if a.eql?(b),
then a.hash == b.hash, but #eql? itself should not *depend* on #hash.

For example, given

  class Thingy
    def eql?
      instance_of?(other.class) && hash == other.hash
    end

    def hash
      [name, *tags].hash
    end
  end

if #hash produces a collision for different values of [name, *tags], two
Thingy objects will appear to be eql?, even though this is not the case.
Instead, #eql? should depend on the equality of name and tags directly.

Closes Homebrew/homebrew#20716.

Closes Homebrew/homebrew#20797.

Refs Homebrew/homebrew#20829.

Now that --build-bottle is filtered we don't need to worry about extra
installation time.

Fixes Homebrew/homebrew#20682.

References Homebrew/homebrew#20807.

Closes Homebrew/homebrew#20774.

Refs Homebrew/homebrew#20819.