add save_dataframe() method to sqlite wrapper; remove Fetchers save() and load() methods

[mageirakos]

Motivation

Upon review the save() and load() methods inside the Fetcher classes are redundant.

Modification

1. Add a save_dataframe() method to our sqlite wrapper which in conjuntion with the get_dataframe() method
   achieves the same results.
2. Modify classes and the fetch_docs/fetch_issues scripts to work as intended