Add pbjson feature for Protobuf JSON serde

[mbrobbel]

This adds an optional pbjson feature that uses pbjson to add Serialize and Deserialize implementations based on the Protobuf JSON mapping.

I've also refactored the build.rs file to use the walkdir crate (instead of glob), and use std::path types instead of strings.

This builds on #5 and relies on influxdata/pbjson#67, so marking this as draft for now.

[jvanstraten]

I don't see anything weird with this, nor any reason not to include pbjson support. @andygrove?

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[andygrove]

Apologies for the late review. LGTM.