Log messages routed to stdout/stderr as appropriate.

This also uses a dedicated logger instead of passing everything to the
root logger. `overviewer_core` still needs to be updated to reflect
this.

Additionally, `logger.configure` has been simplified.
Using a dedicated logger inside overviewer_core.
docstring for InverseLevelFilter added.
Removed second logger.

The root logger for "overviewer" is now named "`overviewer_core`".
Fixed log format reconfiguration regression.

Commit c991135 introduced a regression where it was no longer possible
to re-configure the loggers once they had been set up. This is now
fixed.
Fixed a crash when redirecting stdout.
Fixed the way texture.py searched for foliage, grass and chests textures.
This is a "quick-and-dirty fix", you may improve it as I'm novice in Python.
Default files in data/textures have been renamed to reflect changes in new 1.6 texture packs (aka resource packs)
Merge branch 'master' of git://github.com/overviewer/Minecraft-Overviewer
Adding blocks for new flowers and doublePlants
Add new grassless dirt and podzol block types for 13w36
add packed ice block for 13w36
Refactored stairs rendering to support corners
Merge Pull Request #987
Merge Pull Request #986
Added some icons for people to use in their POIs.
Moved icons to subdirectory, adjusted docs

I also added one of the icons to the manualpois example, so people
have an exmaple of how they can use these in their configurations.
Added genPOI workaround for MystCraft

Possibly fixes issue #993.
Fix error reporting in genpoi when a bad dimension is specified
add red sand. move packed ice so blockIds are in order
Update config.rst

Fix typo in example code that causes error in an actual config file.