Fully separate concerns between Scenario and Runner

[benlangfeld]

WIP

• [x] Runner should take a pre-built Scenario
• [x] Default to using temporary files for generated scenarios unless they are explicitly compiled to disk
• [x] Allow SippyCup to execute SIPp scenarios using a YAML manifest (replacing steps with embed or similar, passing either filenames or embedded XML scenarios).

[benlangfeld]

• [x] Scenario name should have a default if one is not supplied
• [x] Scenario name should be implied from manifest filename where possible

[bklang]

Overall this looks pretty good. Can I also get a Changelog entry?