Add SagePay Direct PayPal integration

[bullenb]

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This is a pull request that adds the PayPal integration that was built as part of the oilstone/omnipay-sagepay fork, however that fork cannot be merged back in due to changes in the composer package definitions.

This is still a work in progress and will still need to be tested but it is a start to adding proper PayPal integration into this package.

[bullenb]

Still a work in progress. No tests have been set up.

[bullenb]

Some typo fixes have also been included in this pull request.

[judgej]

I've not managed to have a good look at this one yet, but it looks like the test failures are just formatting issues - missing terminating line endings on your new files.

[eileenmcnaughton]

I pulled out a few on these into a separate PR since this would be more readable with less changes & there is a lot of unrelated cleanup in it https://github.com/thephpleague/omnipay-sagepay/pull/162