Improvements of plots and addition of todo plot

Changelog:

in keystrokes.R

- Started saving to svg (vector graphics better than raster plots).
- Changed polar plot titles.
- Suppress y-axis value labels in polar plots (counting 16000 minutes is no longer meaningful, it's the pattern that is interesting).
- Slightly increased font size for some plots using `theme_minimal(base_size = 16)` argument.
- Changed location of legends to improve use of space.

in todos.R

- Added a simple todos plot for now.

in README.md

- Added reference to todo plot.
- Changed links to refer to new .svg plots.

Changes to be committed:
	modified:   README.md
	modified:   plots/keysOverTime.png
	deleted:    plots/polarAll.png
	new file:   plots/polarAll.svg
	deleted:    plots/polarSplit.png
	new file:   plots/polarSplit.svg
	deleted:    plots/polarSplit2014.png
	new file:   plots/polarSplit2014.svg
	new file:   plots/todos.svg
	modified:   scripts/keystrokes.R