Populate top-level changelog from repo history & docs info

[sadielbartholomew]

Close #19, in particular to add a single changelog with:

• a history with time order reversed relative to the docs 'Planned releases' page so that the more recent releases are listed first rather than last;
• release dates are included (which I've pulled from the repository 'Tags' page for lack of other information in this respect).

Still TODO for the changelog:

• Add in all of the more detailed version notes from the http://ajheaps.github.io/cf-plot/version_<version_number>.html pages, available for later versions (1.3 - 3.3), then remove those as separate pages.

However, due to the urgency of getting this lone changelog up, so I can start working on the codebase and keep track of any updates using it, I will leave the above TODO for a later time.

Non-controversial admin PR, so no peer review required.