Slightly improved version of lelit's #172

This means the eggs/ cache dir creation also moved to the start of buildout
instead of happening somewhere deep in the installation mechanism. I'll need
to adjust the test output for this.

Avoid prefixing buildout dir to 'special' rpath elements.

Noted the change in buildout's changelog, too, as that's much more prominent
and as it fixes a buildout bug.

Travis speed improvement through caching

The [:] is necessary to edit the os.walk results in-place. Otherwise the
filtering-out of .svn, CVS and so doesn't work as intended.
(Note: this regression I put in was caught by one of the tests! Good that
we've got such a large amount of tests!)

Added fs_to_text() function that ensures a filename is always unicode (python
2) or str (python 3).

os.walk() gets such a unicode dirname as start point, so that it tries to do
its best on python2. The resulting filenames are converted to unicode to make
really sure that the join() doesn't fail with a decoding error.

The input to the hash function is encoded, as it gives an ascii encoding error
otherwise on python 2.

(Good test to see if it improved travis' speed!)

The cache mechanism of travis-ci.org itself creates the cache directory, so it
is already there before we try to add it.
The bin/python inside it seems a good target.

Disabling sudo enables the use of the container based infra, necessary for
caching.
The directory with the downloaded and compiled python is cached.

Use local ez_setup.py if one exists; add --setuptools-download-base option

The pull request itself contains a change that should not be applied.

Run easy_install with specified setuptools_version, same change as #232 

Run easy_install with specified setuptools_version

Use 'rcN' for release candidate versions in tests

Our VersionConflict handling could result in an UnpackError when rendering