Add project euler 19 solution

[soaibsafi]

Describe your change:

Add a new solution for Project Euler 19 based on Zeller's congruence.

• Add an algorithm?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.

[dhruvmanila]

Announcement:

@soaibsafi

This message is sent to all pull requests which are OPEN, NOT MERGED, not containing 'wontfix' label and contains 'Project Euler' or 'Project' or 'Euler' (case doesn't matter) in their title. If this message doesn't apply to your pull request, please ignore this.

Message:

This is to notify all the users submitting a pull request for the Project Euler solution that the README.md file has been updated with all the style requirements necessary for the directory and solution files. Please read through it and make all the necessary changes required to your submission.

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

Please reopen this pull request once you commit the changes requested or make improvements on the code. If this is not the case and you need some help, feel free to seek help from our Gitter or ping one of the reviewers. Thank you for your contributions!