This fixes some layout glitches with smaller graphs, and also
improves the convergence speed.

This implements a graceful fallback in case GTK cannot be loaded, which
can (currently) happen simply by not having a environment DISPLAY
variable.

Ressurect old edge_reciprocity code.

This install the boost workaround headers together with graph_tool's
header files.

This resolves ticket #100

This makes the code more efficient if the input parameter combinations
are much larger than the graph itself.