* Avoid n^2 complexity during decoding (#12)

`push` using `Array.concat` creates a new copy of the array for each
inserted element and leads to n^2 complexity when decoding arrays.
For pprof with large numbers of samples, this quickly leads to very long
decoding (>1 min) compared to ~1s with this change.

* Add some interoperability tests with protobufjs (#13)

Check that profiles encoded with pprof-format can be decoded by
protobufjs and that pprof-format can decode profiles encoded with
protobufjs.

* v2.0.7