The build directory cannot contain subdirectories

[guewen]

Hi,

The build directory is usually 'docs'. However, I have a buildout which may have many documentations (1 documentation per OpenERP addons), so I need a build directory as follows (each documentation has its own buildsphinx command line):

docs/addons1
docs/addons2
...

Actually, we can't use sub-directories in the build option.

This is because the build directories are created using os.mkdir, I propose to use os.makedirs (I'm doing a PR).