Gracefully ignore WebKit import until such time as it's actually needed.
Merge pull request #31 from jamalex/ignore_webkit_import

Fixed #26 - Gracefully ignore WebKit import until such time as it's actually needed.
Updated Cocoa backed to use startup callback.
Merge branch 'master' into start_sequence
Updated GTK backend to use startup callback.
Ensure that app and window instances are correctly assigned to all widgets.
Updated iOS backend to use startup callback.
Updated win32 backend to use startup callback.
Updated main docs to clarify when GTK is used.
Updated tutorial to use new startup structure.
Refactored platforms into their own separate repositories.
Bumped version number for release.
Back to development mode.
Simplified end-user dependency to be just 'toga'.

Specific backends can be installed using toga[backend], but by default, installing just
toga will pull in the most appropriate backend for that platform.
Fixed #20 -- Ensure identifaction of all Linux platforms.
Improved implementation of set_platform().

These changes allow set_platform() to be invoked with no argument (to allow
'reset to system default', removes the need for an explicitly declared locals
variable, and allows any external user to manually specify their own backend.
Fix web browser tutorial code.
Merge pull request #35 from readthedocs/patch-1

Fix web browser tutorial code.
Updated tutorial-1 formatting
Merge pull request #36 from jefftriplett/patch-1

Updated tutorial-1 code formatting