Add store transaction logs to CSV export & points remaining report to coach reports 

1. Fix https://github.com/learningequality/ka-lite/issues/2492 by adding an option to export store transaction logs
2. Fix https://github.com/learningequality/ka-lite/issues/2493 by adding a simple 'points remaining' coach report 

@aronasorman Should be reviewable/mergable now if in a rush, but still on TODO list: 

* add tests for simple coach report 
* add tabular report navigation bar to store log coach report 
* add a 'detail view' for store transaction logs (when clicking on student, show what they have purchased in a detailed coach report 

![screen shot 2014-10-17 at 10 22 45 am](https://cloud.githubusercontent.com/assets/1319950/4682948/4502b040-5622-11e4-8ab6-60fa154a2086.png)
![screen shot 2014-10-17 at 10 22 54 am](https://cloud.githubusercontent.com/assets/1319950/4682949/4507a992-5622-11e4-9b90-c82b71c7e714.png)

I guess this is just a shortcut for `CreateStoreTransactionLogMixin`.