[Cmake] Added ForEach command

[Ken Martin]

I added a for each command, only tested on linux so far. For
example...

FOREACH ( tfile
        ContinuousClose2D
        TestCityBlockDistance
        TestAnisotropicDiffusion2D
        TestAnisotropicDiffusion3D
        TestFFT
        TestAccumulate
        TestAllBlends
        TestAllBlendsFloat
        TestAllFlips
        TestAllLogic
        TestAllMaskBits
        TestAllShrinks
        TestGradient
        TestGradientMagnitude
        )
    ADD_TEST(${tfile}-image ${VTK_EXECUTABLE}
        ${VTK_SOURCE_DIR}/Common/Testing/Tcl/rtImageTest.tcl
        ${VTK_SOURCE_DIR}/Imaging/Testing/Tcl/${tfile}.tcl
        -D ${VTK_DATA_ROOT}
        -V Baseline/Imaging/${tfile}.png
        -A ${VTK_SOURCE_DIR}/Wrapping/Tcl
        )
  ENDFOREACH ( tfile )