Add `_conversationAdded` and `_conversationRemoved` hooks. 

Support _rtmClientSign hook. 

creating conversations supports `is_unique` 

Features: - Support Android Key (`LEANCLOUD_APP_ANDX_KEY`) in LeanEngine environment. - push.send now supports new parameter `flow_control` for smooth push.

Merge pull request leancloud#424 from weakish/release-2.2.0 docs: release notes of 2.2.0

🔖 release 2.1.14 

🔖 release 2.1.13 

Fixed - Push: expiration_time - Push: updated_keys - LeanEngine: X-LC-Prod request header

Merge pull request leancloud#401 from leancloud/develop/smiao 2.1.11

🔖 bump version