Added property getting company executives

[nikolamilosevic86]

I have added a functionality that reads company executives with their salaries and dates of births if available. The retrieved information comes in dataframe in property executives.

[ProgrammingDao]

Happy to help with the review if appropriate.

[ValueRaider]

Hi Nikola. I'm willing to merge but have one request - move the fetching into it's own function e.g. Ticker._get_executives(). Each scrape url should have its own function.

[fredrik-corneliusson]

@ValueRaider if merged please make sure it does not affect performance of other API:s
New features should be carefully added and be of common use as all new features need to be maintained and supported.

In my opinion before adding more features to the already bloated and buggy base.py the code need to restructured to allow for a more plugin oriented API.