Add the --allow-hosts=None argument to easy_install.

This forces easy_install to use its local cache of egg files when installing,
rather than downloading from PyPI, and protects us from the PyPI server going down.
Adding local copy of bootstrap.py to reduce dependencies on external servers.
Further reductions of external dependencies: use local copies of ez_setup.py and distribute_setup.py
Checking in the downloads cache, and editing Makefile and buildout.cfg to force its use.
Add local copy of the zc.buildout egg
Update build docs: remove steps that depend on PyPI, and add XML library dependencies
Point easy_install at the correct cache dir to allow dependency resolution
Reverting checkin of downloaded files
Put the buildout_downloads cache in /tmp, so that downloaded packages persist between builds.
Add wrapper script around bootstrap.py to provide nicer error message
Remove download-cache setting, so that build servers can override it
Removing obsolete file
Reverting further obsolete changes
Merge pull request #14 from Metaswitch/use-local-eggs

[Reviewer: Felix] Use local eggs for installing, and improve error messages on build failure