Merge pull request #211 from SrNetoChan/master

New screenshots for 2.2 User Manual
inserted information about gradient fills of polygon layers
extended gradient fill notes
removed error
Typo in Processing manual
Added a chapter on other backends (to be done)
Improvement to Processing manual
Added advanced note to Processing
Added config for OTB
Path for OTB on deb
Update preparation.rst

move https://github.com/qgis/QGIS-Training-Manual/pull/17 to here
Status bar description
Info on Print Composer Options
Update navigation tools
Corner radius for rectangle
Update Composition tab settings
World file for export as image
Add Size and position section with new options
Atlas update and minor fixes
Rotation in common properties