ENH: Make payment identifier length configurable - globally / per gat…

…eway (silverstripe#203)

* Fix payment Identifier collision check and add test
* Make payment identifier length configurable
* Add unique index to Identifier column

Merge pull request silverstripe#198 from silverstripe/wilr-patch-1

Merged silverstripe#198

allow false for `is_offsite` param. (silverstripe#193)

* allow false for `is_offsite` param.
* Fix unit-tests.

Make gatewayinfo respect `false` settings on `is_offsite`. (silverstr…

…ipe#191)

* Make gatewayinfo respect `false` settings on `is_offsite`.
* Fix unit-tests for changes that happened in the Dummy gateway.
* Install from source in travis.

Static route tests and documentation. (silverstripe#179)

* Added `getStaticEndpointUrl` helper method.
Restructured `PaymentGatewayController` code, so that public methods come first.
Added unit-tests to test the new static-route feature.
* Added documentation about static-route feature.

Fix typo in documentation.

Adding changelog for 2.2

Adding 2.1 changelog

Merge branch 'master' into 2.0

Allow failure of tests against core-master