'download-cache' option should create a path if necessary

This causes problems not only for caches under the buildout itself, but even for more modest attempts to avoid hours of repeated downloads, like setting the download directory to ~/.buildout/cache to mirror the way that buildout treats ~/.buildout/eggs.

In the patch suggested,
buildout will recursively create the cache directory if the path does not exist, by using calling os.makedir, which will raise an exception in case of write error.

In case the path exists but its a file a directory, a proper error message will be printed to the user.

Hi,

A month has almost passed since I uploaded the patch.
Can someone please review it and merge it to the source?

Two things:

- It needs a test. :) (Or it needs to update a test if the change
  would cause a test to break.)

- I'm -1 on automaticaly creating the download cache, in general,
  especially with makedirs. I'm willing to compromise. :)

  I'm willing to create the directory if it's in ~/.buildout or in
  buildout-directory. I strongly prefer that it be created only if
  it's imediately in one of these (iow, use mkdir).

https://github.com/Infinidat/buildout/commit/6ff28d490aaf62719534eac8b29994ec7c0a2564

Still needs a test.