Added Integration tests

[aleitner]

No description provided.

[aleitner]

To run mink/behat integration tests, use the command:

source ./integration_tests.sh

You can configure which instance of bitpay.com this will test with to by changing the url
in the behat.yml file. Make sure you replace username and password with the credentials
used to log into the bitpay site you are testing with.

You can also run specific tests from a command like so:

php bin/behat tests/integrations/invoice_create.feature

And you can run specific lines from these tests by:

php bin/behat tests/integrations/invoice_create.feature:20

Tests run individually require you to set environment variables for your bitpay
credentials or they must be set in the behat.yml file.

Also keep in mind that rate limiters may hinder some tests and they need to be
reset every so often.