Added Lerpable interface +
lerp function for classes implementing Lerpable.
Create Headless and Base graphic-related assets

Inherit from BaseFont for common OpenGLFont behavior
Remove less-useful base classes

Make GLSLMaterial inherit from BaseMaterial
Merge pull request #871 from synopia/behavior

Behavior integration improvements
Fixed gathering of sub types from active modules (was not checking the full hierarchy of subtypes)
Merge branch 'develop' of git@github.com:MovingBlocks/Terasology.git into develop

Conflicts:
	engine/src/main/java/org/terasology/engine/bootstrap/EntitySystemBuilder.java
Switched NodesClassLibrary over to using the new findAllSubclassesOf method.
Merge pull request #861 from mkienenb/headless-assets

Create Headless and Base graphic-related assets
Implemented NUI-based pause menu and death screen.
Merge branch 'develop' of git@github.com:MovingBlocks/Terasology.git into develop
Fixed unit tests.
Removed old Pause and Death screens.
Added error message when trying to create a world with no world generator selected.
Added generic linear interpolation objects and functions
Added modulus and remainder to Teramath.
Merge with latest
Checkstyle fixes I should have run at the time of the PR
Added generic linear interpolation objects and functions
Added modulus and remainder to Teramath.
Remove lwjgl dependencies from GUIManager by making it into an interface

Add headless GUIManager
Use proper Terasology icon
Added 16x16 icon

For some reason, it appears to be ignored by .gitignore
Merge pull request #873 from mkienenb/headless-checkstyle-fixes

Checkstyle fixes I should have run at the time of the PR