add support for multiple signature keys

[maurociancio]

Add support to have multiple signature key, according to #206 .

This is useful to rotate keys without breaking existing clients.

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[maurociancio]

@googlebot I signed it

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[willnorris]

This looks great... I love how simple it turned out. A few minor, mostly stylistic, comments below. I'm happy to make these changes myself before merging, but given how long it's taking me to do reviews, that might take longer :)

[maurociancio]

Thanks for merging!