negative int64 values handling is not compatible with reference Protobuf encoding

#14 by @yhahn had a nice int64 decoding/encoding fix, but the resulting byte sequence is not compatible with how it's handled in the reference Protobuf implementation. `int64` are cast into `uint64` using [Two's complement](http://en.wikipedia.org/wiki/Two%27s_complement), so when decoding, each byte should be flipped, and `1` added to the resulting number.

Here's how I hacked this together in my `pbf` implementation for decoding, and it seems to work: https://github.com/mapbox/pbf/blob/int64/index.js#L119-L120

Compare the repository's negative int64 encoding and decoding with the reference Protobuf behavior and the linked pbf implementation at index.js#L119-L120. Confirm that negative int64 values produce byte sequences compatible with reference Protobuf encoding.

Written by the indexing model from the issue text.