Add a Tetragon class.
Merge pull request #6 from tkihira/hbono/add-tetragon

Add a Tetragon class.
Add anchor points and implement multiplication of matrices.
implemented: DisplayNode#setColor
Merge pull request #7 from tkihira/hbono/add-anchor

Add anchor points and implement multiplication of matrices.
implemented: DisplayNode#setAlpha and DisplayGroup#setColor/setAlpha
implemented: optional fix-scaled parameters for image shapes
resolved conflicts
resolve conflicts