Vishnu

**Adds**
- Adds backend for [Google Cloud Datastore](https://cloud.google.com/datastore/docs/concepts/overview)

- Migrates to `pycryptodome` to fix [CVE-2018-6594](https://nvd.nist.gov/vuln/detail/CVE-2018-6594)

**Fixes**
- Fixes timeout calculation by switching to `datetime.utcnow()` instead of `datetime.now()`

**Fixes**
- Fixes issue setting timeout on an already existing session

**Adds**
- Adds support for python 3 33
- Adds support for `pylibmc` backend

**Drops**
- Drops support for `python-memcached` backend under python3

**Fixes**
- Fixes bug when initializing session with a timeout parameter 35