Revamped calculations/aggregations methods

Simplified the way we deal with calculations and aggregation.
I believe we got a little too excited when we started pybamboo and wanted
to add too much value to it.

I think that until we have users' requests and documentation, we should keep close
to the actual Bamboo API.

I thus removed the notion of aggregation since bamboo doesn't use such language.

To work with them, now we have:

add_calculation(name='sum_of_amount', formula='sum(amount)', groups=['food_type,something_else', 'another'])
add_calculations(path='/tmp/my_calcs.json')
add_calculations(json=[{"name": "sum_of_amount", "formula": "sum(amount)", "groups": ["risk_factor", "food_type"]}])
add_calculations(content='[{"formula": "sum(amount)", "name": "sum_of_amount", "groups": ["risk_factor", "food_type"]}]')

remove_calculation(name)
get_calculations()
get_aggregate_datasets # added a get_aggregations() alias for it to match bamboo API name

Simpler right ?

So I removed the following:
* add_calculation(formula) # managing the formula string by hand seems anti-pythonic.
* _split_formula(formula) # dependency of above.
* _is_formula_an_aggregation(formula) # idem.
* _process_formula(formula, is_aggregation=False, as_dict=False) # idem.
* add_calculations(formulae) # used to batch calculation creation as a list of previouly formatted strings. Now we'd just send a list of dicts.
* add_aggregation(formula, groups=None) # was a `shortcut` to calculation with a group. Useless.

Tests have all been adapted and passes.
docstrings for add_calculation and add_calculations
Merge pull request #27 from modilabs/revamped_calculations

qui tacet consentire videtur