First pass at Rtools translation
Prefix rtools path when calling R()
Fixes from Windows testing
Check path exists
Improve documentation of rtools search
Improved tests
Testing rtools path detection
Fix warning message. Add compatibility tst
Make conditional flow clearer
Re-document
More informative diagnostic messages.
Including new check for registry and installed compatibility
Update documentation
Bump rtools 2.16 r max version.
Merge pull request #228 from hadley/winpaths

Improved rtools detection on windows. Fixes #214