IE download fix

[Miroko]

• Fixed download on IE by using FileSaver.js instead of setting href. This adds a new dependency but should help to keep different browsers supported in the future as well.
• Updated jszip to 3.1.3 as https://github.com/Stuk/jszip/issues/53 has been resolved
  • Changed the compression method to DEFLATE instead of STORE
• Also modified the index.html to be a test page for this library.
• Updated readme.

This pr should close these issues: https://github.com/mapbox/shp-write/issues/49, https://github.com/mapbox/shp-write/issues/48, https://github.com/mapbox/shp-write/issues/28

[alexmuro]

This code helped me a ton in chrome to download larger files. Was getting about:blank#blocked with the current download clode.

[sheindel]

Should be resolved thanks to https://github.com/mapbox/shp-write/pull/96 and https://github.com/mapbox/shp-write/pull/100 , will publish soon, thanks!!