Add Virtual file backend support.

[rainwoodman]

TODO:

• Allow overriding bigfile methods, and port existing uses to a C based POSIX backend.
• Add Python correspondance, and PyBackend, URLBackend.
• Clean up the tracing print messages.
• Consolidate the test cases in Python with parametrized test and MPITestFixture, then add tests for None, PyBackend and URLBackend
• Add a file discovery mechanism, test it with the python simple http server.