Updated default build to use Cortex 9.0.0-a10
Moved Plug binding details to anonymous namespaces.
Fixed deadlocks if computes spawn their own threads.

We need to release the GIL on the thread which requests the compute, so that if the compute spawns threads, they can enter into Python.

The only example we have of this right now is the Instancer when getting the bounds, but I've fixed all the ValuePlug::getValue() bindings too for good measure.
Added GafferScene::PathMatcher::prune() method.

This is equivalent to removePath(), but additionally removes all descendant paths.
Added GafferScene::PathMatcher::isEmpty() method.

Fixes #1076.
Merge pull request #1091 from johnhaddon/getValueGILManagement

Fixed deadlocks if computes spawn their own threads
fixed SubTree sets problem where root path has no leading slash
Fixed PathMatcher::prune( std::string ).
Merge pull request #1096 from davidsminor/subtreeSets

fixed SubTree sets problem where root path has no leading slash
Merge pull request #1095 from johnhaddon/pathMatcherAPI

PathMatcher API additions