Update db.php

Added a function "get_event_months" that returns a list of distinct yyyy-mm strings of start_date.
Update filterbar.php

Added switch option "months" to function "show" to add a dropdown with yyyy-mm strings as values to select single months from the filterbar. 
The dropdown is created by the function "show_months" as in function show_cats, just with yyyy-mm as values.

It can be used by setting "filterbar_items=months_dropdown" just like the category-dropdown.
Merge pull request #82 from grosshirn/develop

Added a Month-Dropdown to Filterbar