- small Attribute and InputTypeBase interface changes to make things cleaner
- move html quoting into templates (use ${blah | h} syntax)
- converting input types to use new format.

- add an Attribute class
- input types just need to declare which attributes they want, and how to transform and validate them, and the base class will do all the rest.
- change OptionInput to new format.

- all inputtypes should have id inputtype_${id}
- should have class capa_inputtype

- allows catching any exceptions and making sure the xml is in the error message
- isolates subclasses from external interface a bit

Remove non-working TrueFalse functionality from choicegroup, combine tests with radiogroup and checkboxgroup

Feature/arjun/allow save

Update startup script for 12.10 Ubuntu.

Added "quantal" to the list of known distribution names.

Improve fileinput message

Support for multiple rectangle hitboxes in imageresponse

 - switch to using registry
 - pull math and solution "input types" into a separate customrender.py file
 - make capa_problem use new custom renderers and input types registries
 - remove unused imports, methods, etc
 - add tests for math and solution tags.

- also add a test for chemicalequationinput

- tests
- rename it codeinput, with textbox still supported too

- add test

Unify math and non-math code and templates.

Add tests.

- Make it into a class.
- Combine ChoiceGroup, RadioGroup, CheckboxGroup implementation.  (All three tags still work--this just unifies the code)
- add tests

Fix imageresponse in Firefox