euler 2.0 release

Add new features, include:

1. GQL execution layer;
2. attribute index;
3. message passing interface;
4. more algorithm.

Euler 0.1.2

1. Make use of HDFS optional.
2. Add implementation of (scalable) GNN.
3. Enable use of jemalloc.
4. Optimize speed of loading large dataset.
5. Optimize logging when loading.
6. Enable use of feature in all model.
7. Boost version to 0.1.2 .
8. Other fixes and optimization.