Fix #18

Align the local timezone offset and the timezone offset for new `Date(1990,0,0)`
Don't unconditionally clobber any existing JSZip variable.

This fixes issues where a browser had already loaded jszip.js but isn't
using require. This also bumps the version to 2.3.1.
Tell Excel to refresh formula calculations as soon as the file is opened.

If a formula is provided by JS, then JS should not be expected to also provide the initial result of that formula in order for the spreadsheet to look correct on open. 

The attribute fullCalcOnLoad="1" tells excel that all formulas should be evaluated on open.

Solution taken from:
http://stackoverflow.com/a/18447197/1413853
Merge pull request #21 from eddieloeffen/patch-1

Tell Excel to refresh formula calculations as soon as the file is opened...
Merge pull request #20 from seansfkelley/master

Don't unconditionally clobber any existing JSZip variable.
Merge pull request #19 from eskimoblood/patch-1

Fix #18
Cleanup and check for XML file before read

According to:
https://github.com/porsager/xlsx.js/commit/221711e4c4c25fd27b907aba911e27db28efc4b1

Google Docs does not save the docProps/core.xml file. Added a check for
each XML file's existence before attempting to read.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	package.json
	xlsx.js