Wrap commands in sh -c ''

This pull request wraps commands in sh -c '' so each of them runs in its own context.

It implements what [I have been proposing](https://github.com/drone/drone/issues/562#issuecomment-62550047) in issue #562.

This is mainly for discussion purposes. I have simply implemented the change and fixed all tests to pass. I have no idea if it will actually work of how big the concequences will be... :-)