Added support for env_vars

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>
Added rbenv_env_vars recipe to rails role

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>
Added documentation for this feature

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>
Merge pull request #70 from jvanbaarsen/env_vars

Added support for env_vars