Adding the date and dates properties for documents.
Adding the ability to replace `{date.date_name|format}` style dates in URLs.
Adding a default date format if none is given for the date.
Updating to accept formatting of the default `{date}`.
Merge pull request #24 from Zoramite/feature-dates

Dates in URLs and document meta data
SDK updater checks the latest release from GitHub instead of reading the VERSION file (for #19). Also removes some dead code.
Installer gets latest release from GitHub, instead of hardcoding the version.
Prompt to run auto-installer when local version is behind released version. #19
Bump to version 0.0.28.
The body and html properties of YAML-formatted content documents should always be None. Add documents_test.py.
Handle trailing slash redirects on the development server.