expose functions for deserializing http types from raw event sources

[softprops]

fixes #92

Description of changes:

These changes address the need of a way to deserialize http requests from raw payloads for local testing purposes by exposing two new functions lambda_http::request::{from_str, from_reader}

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[softprops]

@davidbarsky ping

[davidbarsky]

@softprops Sorry, and thanks for this!