fix issue: I91 Eraser paints black after switching
Test modified/added: testSwitchBetweenBrushAndEraser
testSwitchingBetweenBrushAndEraserAndMoveTool added
Merge pull request #104 from Catrobat/I91_Eraser_paints_black

I91 eraser paints black

Implementation: @schmidiM [MS]
Acceptance: @galstc05 [SG]