New package: purr

This forms part of the MeqTrees software suite. Since the main Timba
build process depends on this and the MeqTrees repository is split
into its components anyway, it makes sense to have this as a separate
formula. Installation is still very crude without a setup.py file.
purr: First working version

Now track the latest release as well as HEAD. Add dependencies on
pyqt and pyfits. Patch icon loader to look in common MeqTrees share
directory first. Install Purr icons in this location.
New formula: tigger

This is the standalone FITS viewer from the MeqTrees suite. Do a manual
install of the package part of Tigger to the Python site-packages
directory, while bin and docs go to the usual locations and the icons
go to a common MeqTrees share directory. The two sets of patches get
rid of an X11-specific method call and make all Tigger imports
absolute.