specify auto_capture? #=> true for stripe and eway

As of spree 1.3, auto capture is determined by
PaymentMethod#auto_capture? (see spree/spree@769c1aa)

This commit makes eway and stripe always return true for auto_capture?,
removing the need for additional configuration on those gateways.

Fixes #55
remove misleading comment in samurai gateway

Presumably this was copied from stripe. The samurai gateway supports
authorize/capture, so there's no need to set auto_capture.