calculator.py: fixed mod error, new features and code changes

[shreydan]

• standalone main() function
• round to nearest integer feature.
• converting degrees to radians feature
• moved string-handling part of result() function to calc() function

[shreydan]

@yadneshk Please always check if the program is working or not before you create a pull request. Issues in your pull:

• mod is not a python math function.
• Indentation error.

Fixed the issues along with my changes. Thanks for adding the mod function.

[shreydan]

Please also take a look at shreydan/python for more of such programs