Support for babelDir != "~/.babel" (for nimrod-env)

This change may be controversial - it checks if symbol `nimrodEnv` is defined and if it is assumes that babelDir is relative to babel executable.

Merging this PR depends on whether Babel is going to more closely integrate with [nimrod-env](https://github.com/zielmicha/nimrod-env). If not, more general way of passing paths may be better idea.