Update parallel.rst

Added information on how to configure ``mem_free`` as a consumable resource, as by default the ``mem_free`` option only checks available memory upon spawning the engines. If we had a host with, say, 16 slots and 60 GB of memory and each job required 16GB, SGE would still happily spawn 16 engines on the host if ``mem_free`` wasn't a consumable.
Support None/none as valid inputs for empty values in input YAML. Enables cleaner Galaxy tool support
Merge pull request #430 from mjafin/patch-3

Update parallel.rst
First draft of Scalpel integration
mutect.py and scalpel.py bug fixes for integration with bcbio
mutect.py small fix to assume two separate callers always used
Merge branch 'scalpel' of https://github.com/mjafin/bcbio-nextgen into scalpel