Add shortcut methods to set entry provider

[stefanuebe]

The Vaadin Grid has setItems methods, that shortcut the creation of data providers. Such kind of api could also be applied to the FullCalendar, e.g.

• setEntries(Entries... ) would create an in memory eager variant
• setEntries(boolean, Entries... ) would create an in memory lazy or eager variant depending on boolean
• setEntries(callback, callback) would create a callback variant

We could also think about implementing the HasItems interface additionally.

[aetasoul]

Integrated the first two d92169e4