Update to JSZip 2.1.1

[dduponchel]

This pull request :

• upgrades the code to JSZip 2.1.1 (.data -> .asText())
• removes the old JSZip files and uses the npm package
• uses nodejs Buffer instead of binary string (since 2.0)

Fix #8

[daraosn]

Sorry the delay, I was on a trip. Code is good, tests pass. Thanks for taking the time to update :-)

[dduponchel]

Thanks :)