Explain is_package AttributeError in find_package

When a PEP 302 import hook is used that doesn't implement .is_package()
an AttributeError is raised. This looks like a bug in Flask. This change
fixes that problem in the sense that it explains, that the
AttributeError is intentional.
Merge branch '0.10-maintenance'
Fix typo in docstring
Merge pull request #840 from mhils/master

Fix typo in docstring
Fix typo
Merge pull request #844 from adamobeng/patch-1

Fix typo
Removed g object from example as it was unused
Rewrote tutorial to use the g based appcontext object
Merge branch 'master' of github.com:mitsuhiko/flask
Some cleanups