Make the SPREADSHEET_CACHE_TTL configurable in the tarbell_config.py file. This setting tells tarbell how long to cache google spreadsheet data. It's useful when google docs is having trouble.
Merge pull request #97 from ryanmark/0.9-beta5

Make the SPREADSHEET_CACHE_TTL configurable in the tarbell_config.py fil...
refactor opening settings.yaml in tarbell configure, fixes #103