Add ReadByte method, satisfies the io.ByteReader interface

[buth]

Hey all.

In this PR I've added a ReadByte method to Reader in order to implement the io.ByteReader interface. The reasoning is that snappy's reader is already buffered, and needing to wrap it with a bufio.Reader instance in order to, say, directly decode varints leads to extra copying and complication.

I've tried to add as little code as possible, and the only notable change to the code of Reader was to move the buffer-filling loop from the Read method to a bufio-style fill method. The test is a modified version of the test for a buffered Writer, but covers the use case of encoding the length of entries as varints.

Thanks!

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[buth]

I did not realize this was a project "owned or managed by Google" (as it's under golang, not google). I'll need to consider the terms of CLA and get back to you.

[buth]

@googlebot I signed it!

[googlebot]

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