Add some standard time sources, and remove the miscellaneous trait

This primary motivation for this pull request is to provide a nice default time source for execution, and package the time source functions up a bit more nicely for the maestro execution api, while maintaining backwards compatibility for the old job api. However, it also closes #238.
It's backwards compatible ... I have raised an issue about it here: #265. Am curious what other people's thoughts are (should we change it or not?)