Mention register_error_handler in errorhandler doc

The documentation for @errorhandler gives "app.error_handler_spec[None][404] = page_not_found" as an example for adding an error handler without the decorator.  However, register_error_handler appears to be the correct way to do this (added 0.7), and it eliminates the problems with modifying error_handler_spec directly.
Add Flask.config_class feature
Add ability to config from a JSON file
Update blueprints.rst - Error Handlers

Added information about custom error handlers in Blueprints with example based simple_page.
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'
Update sessions.py

get_cookie_path: fix docstring
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
Add a non-decorator version of the error handler register function

The main application object has a register_error_handler function
which mirrors the decorator's functionality. According to the principle
of least surprise, make sure blueprints also have this convenience function.