Add tick marks to plots

[andrewdsen]

Provides functionality for tick marks (issue #328).

• Tick marks are toggled with the theme keyword tick_color.
• Tick marks are drawn over grid lines and have the same line width as grid lines.
• Tick mark length is set to 1% of the maximum axis length. (It would be easy to allow user definition via theme if that functionality is desired).

These look best when panel_stroke is defined. On that note, I also added a line width for the panel stroke (set to theme.line_width). This doesn't seem to affect things in the SVG but it makes a big difference in appearance in PDF rendering, where the panel line width is somewhat chunky.

I should note that the dynamic behavior in the SVG output is somewhat broken, in my opinion: ticks move when the window is panned but only the original tick marks are retained (none appear to be generated when zooming). Not sure what to change to remedy this.

[bjarthur]

@andrewdsen can you please rebase this PR with master? i'd like to try out tick marks. thanks.

[Mattriks]

also now #1047. Perhaps time to create a new branch, and modify/update the code in this PR.