Externalize property maps by default

This commit removes the internal property maps from the GraphInterface
class, and makes all property maps external by default. The internal
property maps were moved to the python layer.
Propagate external property map usage to all algorithms
Port clustering algorithms to new interface

A new 'clustering' sub-module was created.
Delete graph-tool script

No longer functional or relevant.
Remove internal properties from run_action code
Fix automake files
Correctly normalize betweenness centrality

Betweenness centrality should be differently normalized for undirected
and directed graphs, i.e., (n-2)*(n-1)/2 vs. (n-2)*(n-1),
respectively. Also, edge centrality should be respectively normalized
with n*(n-1)/2 and n*(n-1).
Correctly normalize betweenness centrality

Betweenness centrality should be differently normalized for undirected
and directed graphs, i.e., (n-2)*(n-1)/2 vs. (n-2)*(n-1),
respectively. Also, edge centrality should be respectively normalized
with n*(n-1)/2 and n*(n-1).
Fix bug when copying graph with filtering disabled

Also, get rid of g.underlying_graph(), and rely on "private" variable
g._Graph__graph instead.
Install missing decorators.py
Improve run_action module

Use tr1::tuple instead of boost::tuple to get around maximum tuple size
limit (this requires GCC 4.3). Introduce get_prop<> template function to
facilitate access of property maps from inline C++ code.
Fix graph copying and filtered graph copying semantics

Graphs are now exactly copied, including filter state and filter
property maps.
Fix OpenMP reduction problem
Fix deadlock in random graph generation

Fix possible deadlock condition in random graph generation with
no_parallel and/or no_self_loops.
Add graph_tool.draw module

This adds a graph_draw() functions which draws graphs in variety of
layouts and formats, and also to the screen, using the python graphviz
bindings.
Fix possible circular reference with property maps

Use weakrefs to keep track of known property maps in graphs, and owner
graphs in property maps to avoid circular references, which may lead to
memory leaks.

This also fixes a vertex removal bug.
Convert histograms counts to float64 by default
Fix problem with ugly type names in run_action
Fix proper handling of properties in graph_draw
Avoid unnecessary string assembly inside inline()