Merge pull request #15 from academe/issue13

Issue13 support user fields

Issue #10 support symfony/property-access v3 or v4

Travis config fix.

Example of a customerDriversLicense that gets through validation.

This is validation at the gateway rather than locally. The documentation
is lacking on this field and its values, but it does seem to be US-only
driving licence details.

Merge pull request #6 from ptuchik/patch-1

Missing import for `Omnipay\AuthorizeNetApi\Traits\InvalidRequestException`

Fix capitalisation in travis link.

Issue #2 Add notification handler.

Add insallation instructions.

Added void and refund functions for credit card payments.