Change image link (now relative)

https://github.com/blog/1395-relative-links-in-markup-files
A horizontalOffsetPlane for things like train tracks and whatever.
Tweaks/extends sky rendering and adds new sky renderings for day and night (I will add high-res versions of the skies as soon as they are finished).

- Adds a new properties to the "sky" shader
- Adds new highlight for the moon (separate from the sun highlight)
- Removes sky cube maps and replaces them with spherical mapped 2D textures
Adds fading for the sun and moon highlights in the "sky" shader.
Adds new high-res day sky.
Adds new high-res night sky.
*  made plants replaceable
*  made water and lava replaceable
-  removed shapes from bricks, allowing to use them as slopes/steps as well
* tweaked the ItemSystem to be able to replace targetable blocks (like plants that can still be broken "by hand")
Merge pull request #441 from skaldarnar/develop

Some minor tweaks to block placement handling and some blocks.
Adds some minor tweaks.

- Adds a new property that can be used to set the blur intensity on the sky (current defaults to 100%)
- Fixes that motion blur with a blur intensity of 0 was active but almost invisible
- Adds various other properties to the Post Processing Renderer
- Adds new enum for various FBO types (more to come) to the Post Processing Renderer
Halves the amount of samples used for the light shafts post processing effect.
Adjusts luminance.
Merge remote-tracking branch 'movingblocks/develop' into smallBugfixes
Added some extra checks into StringMapType serialization, so that null values are not serialized.
Fixed BlockChangedEvents not being sent when the player places a block.
Adds some more properties and tweaks all things related to HDR rendering (in combination with the recent post-processing effect additions).
Increased initial distance for dropped blocks and made inventories persistent for dropped blocks.
* increased distance to 2, blocks thrown away while looking down are no longer picked up directly
* removed deprecated method use
* store placedEntity for dropped blocks

Fixes #310
Merge pull request #443 from skaldarnar/develop

Increased initial distance for dropped blocks and made inventories persi...
Adds some more properties to the "sobel" edge detector shader and cleans up some other post-processing related things.
Fix for StringMap serialization.
Merge branch 'develop' of https://github.com/MovingBlocks/Terasology.git into develop
Tweaks various rendering related things and adds a new custom (less repetitive) ocean/water simulation/animation algorithm.

- Adds that the normals for the offset water vertices get calculated using multisampling and differentiating
- Separates grass animation from water animation (water animation is only available via the "Insane" and "Epic" graphics quality settings from now on)
- Adds various properties to control the water simulation
- Adds support for different quality settings for the water simulation (octaves used for the wave generation can he varied)