Fix problem with Firefox

Hi,

   In Firefox 31 (latest version), I canned use the library because it already has the indexedDB var in the window scope. As we are using strict mode we receive the following error:

TypeError: setting a property that has only a getter

This pull request is to fix this exact problem by replacing line 9 by this:

if(!('indexedDB' in window)) {
    var indexedDB = window.indexedDB || window.mozIndexedDB || window.webkitIndexedDB || window.msIndexedDB;
}