Create Agenda model, import task for Philly city council agendas

Created a simple Agenda model and rake task to populate it from mongo database 'opengovernment_import'.

There doesn't seem to be much of an established standard for Agendas, so I'm going with the following:

`meeting_date` date and time of meeting
`name` - meeting title (eg City Council, Committee on Public Property and Public Works)
`location` - In Philly's case, this is just a room number, but we should expect to see addresses
`agenda` - URL and full text scan of agenda PDF (when available)
`minutes` - URL and full text scan of minutes PDF (when available)