This implements a general wrapper for dense_hash_map and dense_hash_set,
that falls back to unordered_map and unordered_set, if sparsehash is not
enabled.

This also implements direct manipulation of OpenMP settings (schedule
and number of threads) via python.

Xin Sun authored Dec 21, 2013 and Tiago Peixoto committed Dec 21, 2013

In case google sparsehash uses a non-standard prefix, e.g., in Homebrew.

Use schedule(static, 100), to reduce overhead with small graphs in the
majority of cases.

Dumping boost::adjacency_list<> improves memory usage by a factor of
two, and also slightly improves performance in some cases.

This enables the inclusion of the non-boost tr1 headers if a recent
enough version of libstdc++ is detected. This als fixes a bug with
clang when sparsehash pulls unwanted tr1 headers.

This enables support for older versions of GCC (>=4.2), by falling back
to the boost tr1 implementation when the GCC version is less than 4.4.

This fixes a race condition when undirected graphs are used, and when
OpenMP is enabled.

Thins changes the graph filtering code slightly to wrap graph types with
GraphWrap, which automatically updates the edge index list when edges
are removed and added to the graph.

This also changes how graphs are passed to algorithms, which is now by
reference instead of pointer. (hence this touches lots of code, but
changes are trivial)

Python binding will follow