added time when a repo was last checked + files changes for every repo

[zsoltfay]

Together with Danut today I did the following:

Worked on the filter_git_commit_data function:

• when iterating the script prints the repo it is currently working on
• this function now adds to the beginning of each .json file the time its specific repo was checked via the script. This is object 0 so commits are still starting from object 1.
• the json files now include a list of modified files for each commit
• changed the indentation for the logic which creates and writes into each json file, so it only writes when all data is present.

Refactored the name of write_main_mk_table and generate_main_mk_table to the proper file name format.

Added try checks to the extract_github_data and create_git_md_table since the first element in each json file is now the 'lastchecked' date instead of starting with commit data because lastchecked doesn't containt keys such as: commit_message or commit_date.

[bccrisan]

LGTM, there are a few KeyError missing from the try-except, those will be added in the next push.