Simplify buildsystem and fix configure errors

I tested two cases:
* Built as Standalone

```
cmake -DUKFTractography_BUILD_SLICER_EXTENSION:BOOL=ON \
  -DSlicer_DIR:PATH=/home/jchris/Projects/Slicer-2-SuperBuild-Release/Slicer-build/ \
  -DEigen_DIR:PATH=/home/jchris/Projects/Eigen-build ../ukftractography
```

* Build as Slicer extension

```
cmake -DUSE_VTK_6:BOOL=ON -DVTK_DIR:PATH=/home/jchris/Projects/Slicer-2-SuperBuild-Release/VTKv6-build/ \
  -DITK_DIR:PATH=/home/jchris/Projects/Slicer-2-SuperBuild-Release/ITKv4-build \
  -DEigen_DIR:PATH=/home/jchris/Projects/Eigen-build/ \
  -DDCMTK_DIR:PATH=/home/jchris/Projects/Slicer-2-SuperBuild-Release/DCMTK-build/ \
  -DSuperBuild_UKFTractography_USE_SYSTEM_teem:BOOL=ON \
  -DVTK_SOURCE_DIR:PATH=/home/jchris/Projects/Slicer-2-SuperBuild-Release/VTKv6 \
../ukftractography
```

Note: In this case, I passed path to existing projects to avoid waiting for all the sub projects to build.

@hjmjohnson Let me know if it works for you.