Adds a new class called `Tab` that acts as a recipt for install options. A
`Tab` can be serialized to a JSON file for future reference.

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

Also tidied error output. Still could be tidied quite a lot but I am still considering how best to go about it.

Note this only happens if Homebrew is *not* installed sudo. Which is most of the time true.

Fixes Homebrew/homebrew#6899

Rationale: people break their installs by mixing sudo and non-sudo usage. They are used to having to `sudo port install` and shit like that, so I understand why the mixing happens.

* may be a problem if the constant is scoped (like A::B), but this is not the case
  (const.split('::').inject(Object) { |c, part| c.const_get part } would solve that)
* this was noticed in a comment of 9afc85ad but it has disappeared

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

Because we should always be able to help. And I've checked, it works fine.

Because `gem instal` works too.

The code was sucking. To the extent that maintenance was hard. It's a lot
easier to work with code that is sensibly split at sensible boundaries. So
now it is more like that.

But the refactor is minimal. Because we don't want you to have more merge
hell than absolutely necessary.

If you merge you will need to pay attention to brew.h.rb (as it is deleted)
and bin/brew (as command logic is gone). It will be painful, but you will just
have to help git out by moving any changes around manually.

Note compatibility.rb. It ensures that any function renames or removals don't
break anything. We're pretty serious about backwards compatibility. And that's
because we encourage you to hack around with the innards. And we couldn't do
that if we would then just make stuff disappear behind your back.

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

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

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

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

HOMEBREW_CACHE isn't really helpful in diagnosing problems, and some
people don't like their username / home folder going into bug reports
on the web, so let's take it out.

Passing --installed will limit the list to formulae that you have installed.
Documented this and "brew deps --1" which limits the dependency list to
one level down (direct dependencies).

optparse adds an "options" method to ARGV, and so does Homebrew.
Rename this method (and remove optparse blocking script) so that
Homebrew plays nicer with external Ruby software.

This fixes the issue where "gem install thin" would break
"brew server", for instance.