ENH: Add support for spm MCR configuration via environment

This patches add support for enforcing and configuring the spm MCR standalone
version via environment variables. The respected environment variables are:

  - FORCE_SPMMCR: any value, sets `use_mcr` to True
  - SPMMCRCMD: command to use when running spm MCR, if `use_mcr` is True

See also: https://groups.google.com/forum/#!topic/nipy-user/3woo0zStl3M

Additionally, this PR contains two small code improvements in separate commits.
I can remove them, if you want me to.