Addresses #28, #33, adds month outlining

[thismatters]

Please note that I've changed the README to pull the image from my repo, it might be good to change it back if you're going to accept the PR.

This addresses #28 by moving the heatmap down and to the right within the svg so that the legends don't overflow, it also dynamically sizes the svg based on the date range

This addresses #33 by allowing an endDate to be provided. When omitted the startDate and endDate are chosen to bracket the data.

This adds an outline around each month (which can be disabled using .monthOutlineEnabled(false)), which was adapted from here.