Etcd3-py

- merge pull request [90](https://github.com/Revolution1/etcd3-py/pull/90) Fix lease util keeping problems
- merge pull request [89](https://github.com/Revolution1/etcd3-py/pull/89) Add range end and lease to txn
- merge pull request [87](https://github.com/Revolution1/etcd3-py/pull/87) Add handel null value as gogoproto does while modelizing response data
- merge pull request [82](https://github.com/Revolution1/etcd3-py/pull/82) Fix watch util issue 18 and 78
- merge pull request [79](https://github.com/Revolution1/etcd3-py/pull/79) Improve etcd comapabitity of multiple versions
- merge pull request [51](https://github.com/Revolution1/etcd3-py/pull/51) Add a base EtcdModel to all dynamic created model
- merge pull request [42](https://github.com/Revolution1/etcd3-py/pull/42) Improve etcd comapabitity of multiple versions

- merge pull request [34](https://github.com/Revolution1/etcd3-py/pull/34) enum34 only where it's needed

- better code quality
- support etcd v3.2.2+

- finished lock util

- Add more test
- Add watcher, transaction and lease util

You can try it at dev environment

- Implemented all APIs of etcd3's gRPC-JSON-Gateway
- Stateful utils (Watcher Lease Lock Transaction) are in progress