New buildings only next to streets (#47)

Now you can only build at streets.
 Added population statistics (#40)
	modified:   res/lang/DE.xml
	modified:   src/animation/AnimationManager.java
	modified:   src/game/Game.java
	modified:   src/game/Main.java
	renamed:    src/game/MonthlyTransactions.java -> src/game/MonthlyActions.java
	modified:   src/game/ResourceManager.java
	modified:   src/gui/GUI.java
	modified:   src/objects/BigHouse.java
	modified:   src/objects/House.java
	deleted:    src/objects/stadtzentrum.mtl
	deleted:    src/objects/stadtzentrum.obj
	modified:   src/res/lang/DE.xml

	src/gui/GuiGraph.java
	src/gui/GuiQuad.java
Added population statistics (#40) [GitHub failed]
Redesigned delete mode button (#48)
Prepared 10 new buildingtypes (#10)
Implemented file format version

Version control for saved cities is now available to provide downwards
compatibility.
Implemented supply spreading

Supplies are automatically spread via streets when a building is built
or deleted.
Moved supply spreading in seperate thread
Merge pull request #51 from ForceField-IndieGames/Benedikt

Population statistics, new building types, supply spreading