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.
Ok I undid that change. I don't have push access so please merge if it looks good now.