Change RecoveryHandler to accept and return http.Handler

[tdterry]

This is a PR for the proposal in #78. It makes RecoveryHandler idiomatic by accepting and returning http.Handler like middleware in other popular libraries. The previous functionality is exposed in the new RecoveryHandlerFunc method.

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This change is

[jchv]

I just ran into this problem and would like to voice my support for this breaking change.

If API breaks are a concern, maybe it's time to look into gopkg.

[parkr]

I'm a huge proponent of having something like this! I wrote a custom wrapper earlier today because I was looking for this. I'd be happy to see NewRecoveryHandler used in order to avoid API breakage.