Transform subscription amount always in int.
Implemented cancel subscription. Fixed remove subscription.
Fixed test_create_payment_with_client.
Added test_subscription_cancel.
Merge branch 'master' of https://github.com/paymill/paymill-python into github_master

# By Oleg Ivanov (2) and Aleksandar Yalnazov (1)
# Via Oleg Ivanov
* 'master' of https://github.com/paymill/paymill-python:
  snippets improvements
  snippets added
  Update README.md
Updated snippets.