Fix download() to auto-create missing dirs for WP8

- Cordova RTC work item number: 40668
- Apache Cordova Jira issue: CB-6466
- Markmail reference: markmail.org/message/cxy66pe23oyguoal
- IBM PMR: 73985, 000, 781
- Worklight RTC work item number: 40001
- Backport of 77e83c70dd276cd5be5bcc537b1c8a5ddb9b0d31 to Apache master
- Associated mobile-spec test: apache/cordova-mobile-spec#58
- Full description: Changed the download() function to automatically
  create any missing directories in the file path for windows phone 8, to
  bring it in line with the other platforms.