added robot framework contact example

robot framework contact example for android added
Merge pull request #3129 from sebv/http-callback

configurable callback address and port
moving several picker wheels in ios unit tests
Center aligning code blocks for appium.io and added instructions in how-to-write-docs.md
Merge pull request #3135 from moizjv/documentation

Center aligning code blocks for appium.io and added instructions
Merge pull request #3127 from Erolstt/master

added robot framework contact example
Adding selendroid requires internet permission to documentation
Merge pull request #3147 from moizjv/documentation

Adding selendroid requires internet permission to documentation
adding slack travis notification
ci fix
ci fix