Improve vertex text positioning in graph_draw()

This also includes the new text_rotation, and text_offset properties.
Allow empty property maps in graph_union()
Implement edge gradients
Fix issue with graph_union() and filtered graphs
Implement get_hierarchy_control_points()
Improve edge label drawing
Improve self-loop drawing in cairo_draw()
Improve modularity() computation
Modify semantics of prop.fa attribute for edge properties

The filtered array returned now also reflects the vertex filtering, so that
it corresponds only to actually visible edges in the graph, not only
those explicitly selected via an edge filter.
Implement radial_tree_layout()
Refactor stochastic blockmodel inference

This greatly improves the performance of the previous algorithm, and
includes support for hierarchical models.
Performance improvement to condensation_graph()

Avoid hash of pair<vertex_t, vertex_t> types.
Sphinx documentation configuration change