Added keyboard_visible, and keyboard_mode to allow a 'managed' status in TextInput widget
added doc strings
Added show_leyboard and hide_keyboard convenience functions
Removed keyboard_visible BooleanProperty
Merge branch 'master' of https://github.com/kivy/kivy into feature/GeneralStickyTextInput
Merge pull request #1697 from lonelycode/feature/GeneralStickyTextInput

Attempts to fix Issue #1640 - More generalised version of TextInput to add a a"managed" mode