Add cesium point cloud 3D tile format support

[jianhuang01]

for #478

• initial implementation of the tilling tool which supports cesium 3D spec.
• add some improvements to handle app parameters better.

[coveralls]

Coverage increased (+4.1%) to 56.359% when pulling 7d9ddf5 on 3d-tilling-tool into 2331b35 on master.

[xintongxia]

Do you have a screenshot?

[jianhuang01]

Do you have a screenshot?

We didn't support georeference for now, when I am using cesium renderer, the data only show up in the first second, then covered by meshes of the earth. Because of the rendering issue, I can't get a screenshot. But I could see the point cloud is rendered correctly. After adding the georeference next week, we will get better rendering results.