Feature/services pricing service cost graph

* fixed saving costs tree 
* added pricing service costs plugin and unit tests
* added teams, usage types and pricing services to reports
* added forecast to extra costs
* removed percent from DailyCost
* added usages generator

to generate data call:
```
from datetime import date
from ralph_scrooge.tests.utils import generator
generator.usages_generator(date(2014, 8, 1), date(2014, 8, 31))
```

to collect data call:
```
from ralph_scrooge.plugins.cost.collector import Collector
c = Collector()
c.process_period(date(2014, 8, 1), date(2014, 8, 31))
```