fixed crash when strftime returns non-ascii characters

also generally better unicode handling on web_assets (all files are UTF-8)
bumped docs to 0.4. Also lots of docs changes

Added faq entry about exorbitant memory usage

updated features section on the main page

Added a "what overviewer is not" section on main page

put new rendermodes on the options page

re-worded some things under the custom rendermodes section
--version now displays the correct (new-form) version number
added screenshot to the front page.

Also changed "Overviewer" to "The Overviewer"
fixed up comments in googlemap.py, moved region.js to web_assets
dump markers json out in one shot. Added marker comments.
comments clarifications and whitespace adjustments
overviewer no longer trusts ancillary data from blocks that should have none
nbt.py now simply *reads files*, nothing more
Initial commit of the AssetManager

Mostly a blind copy/past from world.py and googlemap.py.  Not runnable

Rewrite Tracking Issue: #565
Scan the FS to find RegionSets

Rewrite Tracking Issue: #568
Moved some more stuff from World to RegionSet

Also, fixed some syntax errors and other minor issues

Rewrite Tracking Issue: #568
skeleton outline of TileSet object and tileset.py created
Added in documetation for what options are accepted
More work on RegionSet and World

Rewrite Tracking Issue: #568
Merge remote-tracking branch 'origin/rewrite' into rewrite
Finished implemention of RegionSet.  not yet tested

Rewrite Tracking Issue: #568
first version of single-process dispatcher

tracker: Issue #564
Removed biome handling and north-direction

We'll have to figure out how/where to put it back in

Rewrite Tracking Issue: #568
Merge remote-tracking branch 'origin/rewrite' into rewrite