Code review request

[GoogleCodeExporter]

Branch name: number_with_unit

Link to the relevant commit(s):

Adds object class and rules.
https://code.google.com/p/oppia/source/detail?r=c984c21fda00ae07d173f739b47e0574
319d9859&name=number_with_input

Merge commit:
https://code.google.com/p/oppia/source/detail?r=534c1e9a45d615779154c2cdaf05e314
81a923e2&name=number_with_input

Fills out front end:
https://code.google.com/p/oppia/source/detail?r=d7391a2338d4f3000d4308b0b412fbb8
57e3283b&name=number_with_input

Purpose of code changes on this branch:
To build the number with unit input feature.

When reviewing my code changes, please focus on:
* What needs to be done next.
* FIXME: Protractor test still needs to be written.
* FIXME: There is an error when the feature is previewed where it indefinitely 
hangs and it displays the schema-format insted of the raw input.

After the review, I'll merge this branch into: develop

Original issue reported on code.google.com by maddenw...@gmail.com on 19 May 2015 at 7:48

[GoogleCodeExporter]

NB: Zoe and I chatted in person today and identified some further changes that 
need to be made in order for this to work properly. I've made comments in the 
individual commits, and would suggest getting a further review before this is 
ready for merge.

Original comment by s...@seanlip.org on 20 May 2015 at 1:06