Addressing Issue#76. Adding start_date, end_date parameters to the _get_json_for_random_dates function.

[anton-shablyko]

Based on: https://github.com/nasa/apod-api/issues/76

Extended function "_get_json_for_random_dates" to accept optional start_date and\or end_date parameters.

If only start_date is provided - the current date will be used as an end_date.
if only end_date provided - 6/16/1995 will be used as a start_date.

The function will raise value error if:

• count number exceeds the number of dates in the provided range
• start date is after the end date

Updated readme.md to reflect my changes.