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
corrected documentation for Request get_json()
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
Fix #856 ommited typo in quickstart
Merge pull request #843 from wbowlin/master

corrected documentation for Request get_json()
Fix with block
Merge pull request #858 from svieira/patch-2

Patch for tutorial (minor code error)
Fix typo.
Merge pull request #863 from jet-sun/patch-1

Fix typo.
Merge branch 'master' of github.com:mitsuhiko/flask