make it possible to define custom REQUIRE_ENVIRONMENT_ARGS
copy setting arg list else same list is used
class based backends for require
change "backend" for "environment" for backwards compatibility
automatic environment detection
Merge pull request #14 from unaizalakain/master

Adding ability to define custom environment classes. Default environment is now "auto", which autodetects based on available environments, with a preference to "node".

Closes #14
Closes #10