Fixes: #156 - Base64 encode and decode algorithms

[augustorsouza]

Welcome to Dart community

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Dart files are placed inside an existing directory (I used the Python directory structure as a suggestion, if it isn't ok please let me know)
• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[augustorsouza]

Fixes: #156

[augustorsouza]

I would love to make this PR count to my hacktoberfest contributions, please :)