Function to generate securitycredential automatically

[dzagey]

I wrote a new function to generate the securitycredential for the user. They now have to declare two more environment variables; MPESA_PASSWORD and MPESA_PUBLICKEYPATH which is used to generate the securitycredential.

The other functions were changed to make the securitycredential param requirement optional and comments and documentations editted accordingly.

This is untested, hope you can pull and test it though and release to the public. P.S. my first public contribution. I welcome feedback. Thank you.

[StanleyMasinde]

Does this project have a maintainer?