Directory restructuring of graph_tool Python module (now a package)

Everything was moved into the graph_tool directory, which should have,
after installation, the following structure:

graph_tool/                          Top-level package
      __init__.py                    Initialize the graph_tool package
      libgraph_tool.so               libgraph_tool submodule
      libgraph_tool.la               libtool file for libgraph_tool
      include/                       C++ include files for dynamically
                                     compiled code