Remove XOR swap from Reverse().

gcc 4.8 on x86_64 produces better code for the naive approach than it
does for the XOR swap trick.
Don't leak globals in test.js
Don't extend Buffer prototype by default.

Make extension of Buffer.prototype opt-in.  The rationale for that is
that some of the buffertools methods conflict with the Buffer methods
of the same name.  Users now have to call `buffertools.extend()` if
they want the old behavior.

Bump the version number to 2.0.0 because this is a breaking change to
the API.