Major and minor x-axis labels

Addresses issue #19 and possibly #25.

Major x-axis labels can be specified in a number of ways:

- `x_labels_major` accepts a list of labels that will be considered major;
- `x_labels_major_every` makes every n-th label major;
- `x_labels_major_count` makes n labels major.

When more than one of these options is specified, one wins. 

Major labels and their grid lines (if applicable) are rendered using the `major` class. The default css sets the font-size for this class to `major_label_font_size`, which defaults to the same value as `label_font_size`. Minor labels are rendered only if the `show_minor_x_labels` option is `True` (which is the default) and do not get any special classes.

I also fixed a small bug where an empty graph with `logarithmic = True` would result in an exception.