Adding gapless ar setting, for gapless invoice numbering

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extra : source : 2482ce245828d74395183c8107bc5912a4ef87ae
fixing test cases re: taxforms

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extra : source : d1616e1c5a08763393cb34bc7da9f4735e0ff90e
Merge pull request #547 from einhverfr/master

Adding gapless ar setting, for gapless invoice numbering