Add Secret Function Support#21

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DavidTPate wants to merge 3 commits intoexpressjs:masterfrom

DavidTPate:master

Commits on Oct 16, 2015

Add the ability to provide a secret function which is invoked with the request object to create a dynamic secret.
DavidTPate
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Commits on Aug 18, 2016

Switch the test example over to a rotating key via a simple `setInterval`. While not the best or most complete example it shows one such method of using this functionality.
DavidTPate
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Remove now unnecessary setting of `Host` header.
DavidTPate
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