Use int64_t instead of int32_t as the default label type.

This introduces support for returning a generator in
subgraph_isomorphism(), so that all matches do not need to be kept on
memory simultaneously.

This fixes issue #255.

This facilitates the development of C++ extensions to graph-tool.

This improves the consistency in the documentation.

This fixes issue #252.

This revers the dependency to 1.54. This can be done since
boost::coroutines has been made optional.

This causes the test to fail when only static variables are available.

This reduces even more compilation time and memory use.

This is essentially because of a Debian bug, where no boost::coroutine
shared libraries are available.

This saves memory during compilation.

This modification provides a significant compile-time memory reduction.