Relicense from LGPL 2.1 to MIT.

This package inherited an LGPL license from Eli Skeggs' upstream,
which had forked Kevin Kwok's upstream package
(https://github.com/antimatter15/bzip2.js), also originally with an
LGPL license.  Kevin Kwok's code was a port of Rob Landley's C
implementation, which was LGPL.

Since then, Rob Landley has switched to "zero-clause BSD"
(http://landley.net/toybox/license.html) when he relicensed his
original code for the toybox project in 2006.  Kevin Kwok switched his
port to the MIT license in 2013.  Eli Skeggs followed Kevin to switch
to MIT, and now we can follow suite as well.

See:
http://landley.net/code/micro-bunzip.c (LGPL)
http://landley.net/hg/toybox/file/37ea9dff9c27/lib/bunzip.c (0-clause BSD)
antimatter15@e2ba119
https://github.com/skeggse/node-bzip/commit/0dd3c810c3b9341957a0a6f638369433b2e68126
yeoman/generator#830 (comment)