ENH/API: Add count parameter to limit generator in Series, DataFrame, and DataFrame.from_records()

When reading data from a generator type collection only reads the firsts number (count) of values.
First step to solve #2305, known the length of the data to allocate memory.

- Add count parameter to Series, DataFrame, and  DataFrame.from_records(). 
- In DataFrame.from_records() deprecate the existing parameter nrows. Count it's more general and only refers to quantity of data units. Also exists on numpy API (fromiter). 
- Some refactor in DataFrame.from_records(). 
- Add tests too.
- Lack of release docs for now.