Revisions to /tutorials/firstwidget.rst
Merge pull request #1136 from Zen-CODE/Doc_English_Improvements_29

doc: revisions to /tutorials/firstwidget.rst
doc: fix macosx installation instructions and mention sub/sup tags in
Label Docs
uix: label. Make sure texture exists before binding
Undoes the strategy of the commit 646aabd;
that commit replaces a feature, this adds one.

Brief update of the module documentation.

Bugfix for F11 key binding.
Bugfix for F12 key binding.
Merge branch 'master' of https://github.com/kivy/kivy into rotateOnLinux