Add additional lines to get autocompletion to work in zsh

Issue #85
initial cut at supporting pluggable auto_sheets
add passing in the current_sheet, in case anybody needs it
remove pathname, went a different route
Warn on auto sheet
added yesterday command
Ruby 1.8 compatibility for yesterday command
Update README with today and yesterday commands
Merge branch 'master' into autosheet_plugins
Refactor of autoloading code for AutoSheets and Formatters
Improve autosheet warning
Add CONTRIBUTORS
Better zsh autocomplete instructions
Merge branch 'better_zsh_autocomplete_instructions'
Fix syntax highlighting
Ported orginal auto_sheet pull request into Timetrap::AutoSheets

Closes https://github.com/samg/timetrap/pull/72