fix(Importer): Account for older browsers properly

* Tests were broken in IE9 and Safari 5 because they didn't support the File API. Made tests conditional so that certain ones only run when the File API is available. If it isn't, a test will make sure the importer is disabled.
* importer.js was using `this.grid.options` when it should have used `gridOptions`, in one case
* `gridOptions.importerShowMenu !== false` evaluates to true when the property is undefined. Changed to use `!!` to coerce to boolean.
* `if (window.File)` and other tests fail in Safari (and possibly other browsers) because the property is undefined. Changed to use $window.hasOwnProperty.