affordability get save and load methods

to deal with serializing to session
add two_applicants to initializer
flatten affordability model initialize method
Shimming D3 with R2D3 for IE<9
back buttons no longer use javascript
step 1 does not need interest rate changer
affordability can load from store when it is empty
affordability saves and loads from session
remove unused methods from refactoring
we now persist params to session
remove save code
Merge pull request #166 from moneyadviceservice/back-session

Fix back button
Merge pull request #164 from moneyadviceservice/d3-shim

Shimming D3 with R2D3 for IE<9
Only load d3 on page that needs it
Merge pull request #169 from moneyadviceservice/optimize-d3

Only load d3 on page that needs it
Shimming base font size for public website mounted version
Remove font size setting from callout container
Affordability page 4
Positioning buttons
Merge pull request #170 from moneyadviceservice/page-4

Affordability page 4