pip install --user fix

[Xender]

Fixes pip install --user failing with "permission denied" error when trying to copy headers (data_files) into destination place.

What failed before: pip install --user copies all Python files correctly to ~/.local/lib/python3.5/site-packages/, but then tries to copy C headers to absolute path pointed by sys.exec_prefix - which still points to system-global directory when installing with --user.

The fix: use relative paths, setup() will take care of them properly.

For reference: https://docs.python.org/3/distutils/setupscript.html#installing-additional-files

Each (directory, files) pair in the sequence specifies the installation directory and the files to install there. If directory is a relative path, it is interpreted relative to the installation prefix (Python’s sys.prefix for pure-Python packages, sys.exec_prefix for packages that contain extension modules).

Also, I needed to manually apply patches from #120 to make it build at all, but for the purpose of PR, I rebased them to master.