Add create_payments method

[amxsv]

Payment of bundle orders in a single request

[nikz]

Hi @amxsv - thanks for the PR!

Could you add a unit test to this? Thanks!

[amxsv]

Hi @nikz, sorry for late response.
I will add it on this weekends or next week.

[armstrjare]

Thanks for your work on this - are you able to add basic tests along the lines of the other create_xxx tests? See the following for an example: https://github.com/xero-gateway/xero_gateway/blob/master/test/integration/create_contact_test.rb