Update documentation
Add changes for Ubuntu 14.04
Enable Ubuntu 14.04; remove QEMU from dependencies

The workflow is now indirectly dependent on QEMU for qemu-img.  A
preprocessing step now handles converting VMDKs to disk images.
Rename by_graph to by_sequence

Writing the documentation, I realized 'by_graph' is really more fit for
talking about multiple-sequence analysis.  'by_sequence' is the right
granularity for workflow results.