factor out overviewer.dat

[brownan]

We want to factor out overviewer.dat. I have started work on this on the "jsondat" branch.

Reason: overviewer.dat becomes corrupt more often than we'd like, and we don't know why. Also, the same information is already stored in easily parsible json format in the same directory, so this file is completely redundant.

Removing this file is also the first step in the quick-scan roadmap from the wiki page: https://github.com/overviewer/Minecraft-Overviewer/wiki/roadmap

This involves 3 steps:

• read in sign data and previous north direction markers.js and overviewerConfig.js respectively, instead of from overviewer.dat
• remove code that outputs overviewer.dat
• Also, I'd like to clean up the relevant code and put all the json related code in a single class, probably in googlemap.py since it already deals in outputting the generated files.

Step 0, which is already done: put markers in markers.js so python knows what portions of the file to pass to the json decoder. We don't want to pass the function call!

[brownan]

This issue is being superseded by the rewrite stuff.