New response codes for transaction initiation

Merge pull request #131 from korzind/PC-479

PC-512 add new error code to public php wrapper

Merge pull request #127 from mschindler83/PC-432-Shipping-Billing-Items

Pc 432 shipping billing items

Merge pull request #118 from cyrrill/paypal-checksum

Reusable Payments and Client Id for PayPal Checksums. Thanks @cyrrill

Merge pull request #125 from enricohofmann/reason

Add reason parameter to library

Merge pull request #126 from pm-alexander-serbe/master

DELETE parameters fixed

Merge pull request #124 from enricohofmann/PR-183

Add Source also 2 Preauthorizations

Merge pull request #102 from tschelabaumann/nebula

4.0.0
Added shipping and billing address
Added shopping cart (items)
Added PayPal functionality
Possible BC break in ResponseHandler.php

Merge branch 'master' of github.com:paymill/paymill-php

Adds code samples for API Reference