New make target for OS X package creation.

Author: Ramakrishnan Muthukrishnan <ram@leastauthority.com>
Signed-off-by: Daira Hopwood <daira@jacaranda.org>
clean target should remove the pkg file.
Distribution.xml file and scripts for building the OSX package.
misc/build_helpers/test-osx-pkg.py: script to test the OS X pkg.

After extracting the contents of the package, this script looks at the
output of 'tahoe --version-and-path' to see if the modules are installed
and invoked from the right path.

Author: Ramakrishnan Muthukrishnan <ram@leastauthority.com>
Signed-off-by: Daira Hopwood <daira@jacaranda.org>
Merge pull request #122 from tahoe-lafs/182-osx-packaging-6

182 osx packaging 6