Skip properties population if missing coreProps, fix indentation

Hi there,

We just started using this handy module, and we just run into a problem: if the docProps/core.xml file is missing (as it is, if you export a Google Spreadsheet as XLSX file), the parse function fails, throwing an error due to the missing file.

To avoid this, I just added a condition, that skips the involved section in case the file is missing. Test added, and some refactoring done here and there. Just moved the whole xlsx.js to 2 spaces (WAY more readable), and split the parse.js test suite into separate functions.