* docs: add guide documents

* docs: update sidebar
Co-authored-by: hongyiheng <hongyihengg@gmail.com>

* feat: support builder pattern

* build: release v0.2.2

* chore: update sign util

* Support keep-Alive Connection
* Rename some classes

* Support multi-type messages #40
* Fix apns field #37
* Add constant packages #42

* feat: update message entities

* feat: update imclient

* feat: update util package

* feat: add error code list

* feat: add constants and update constructors

* chore: change path

* feat: update util packages

* feat: remove json and common-lang3 packages

* fix: ApnsInfo add field

* build: pom version

* Update README.md
Co-authored-by: Yang Libin <szuyanglb@outlook.com>

* fix: user sig in IMClient

* fix: synchronized block

* fix #34  

just so so

* feat: add group api and fix unit tests

* docs: update readme

* feat: implement sns api
* docs: update readme