Rename qt to qt4 (closes #5)

[cartr]

This pull request implements the changes @MikeMcQuaid requests in #5 to ensure this tap doesn't have a name collision with the core formulae (see https://github.com/Homebrew/homebrew-core/pull/8334).

It'll get merged once I get the chance to fully test it.

• [X] brew migrate qt4 and brew migrate pyqt4 with an existing installation of qt and pyqt from the tap result in a working installation.
• [ ] Updating from cartr/qt4/qt correctly installs cartr/qt4/qt4. (it doesn't, see https://github.com/Homebrew/brew/issues/1770)
• [ ] Updating from cartr/qt4/pyqt correctly installs cartr/qt4/pyqt4
• [X] brew install cartr/qt4/qt still does something sensible (it installs cartr/qt4/qt4)
• [x] Fresh installs of cartr/qt4/qt4 and cartr/qt4/pyqt4 work correctly.
• [x] Updating doesn't break Designer and the other built-in Qt applications.
• [x] Updating doesn't break Homebrew-installed software that depends on Qt4 (eg puddletag)
• [x] Updating doesn't break a PyQt4 installation.
• [ ] Third-party software can still build with the renamed packages.

[cartr]

There appears to be a bug in Homebrew (https://github.com/Homebrew/brew/issues/1770) that causes brew upgrade to misbehave if the upgrading user has any formula that depends on cartr/qt4/qt installed. I'll wait until that's resolved to merge this.

[MikeMcQuaid]

@cartr Your HOMEBREW_VERSION is badly updated which is going to cause you a bunch of bugs.

[cartr]

Closing without merging, as now that documentation for different versions of formulae is available I should definitely call this qt@4.