CHIEF Excise measure type fix

This fix is for https://www.pivotaltracker.com/story/show/59551288

Includes two changes:

1. We were asked to show values with 4 decimal points on EXCISE measures. So now we show up to 4 decimal points if available and 2 otherwise (just as it works currently).
2. EXCISE measures "EXCISE - SUSPENSION, 570, REBATED HEAVY OIL, OTHER" and "EXCISE - SUSPENSION, 551, REBATED HEAVY OIL, KEROSENE" should have zero duty rates formatted like this: ```0.00 GBP / Litre (Standard Litre of Hydrocarbon Oil)```. Currently we have ```0.00 % (Standard Litre of Hydrocarbon Oil)```, see https://www.gov.uk/trade-tariff/commodities/3902200010?country=&day=25&month=10&year=2013#import.

This change requires data migrations to be run:

```bundle exec rake db:data:migrate```

After migration run the measures for 
![screen shot 2013-11-08 at 10 35 00](https://f.cloud.github.com/assets/13653/1499156/c67adb60-4850-11e3-9e9a-bcb72dc61d65.png)
3902200010 looks like this: