Add some more styling options

- Add method for setting the typeface.
- Remove the title's top margin so that users can have less whitespace above the calendar if they want. Instead add a nonzero default separation.
- Add option to hide the header row with week abbreviations. In our case we'd like a global header row, not one for each month.
Add back the top margin on month titles

And remove some separator code which is no longer needed now that titles have top margins by default again.
Add back setDivider(null), setDividerHeight(0)
Merge pull request #148 from dlubarov/master

Add some more styling options