attachments

;-)

attachments work now!

things included in this PR:
- support for adding offline attachments in the offlineFeaturesManager.js module
- a working attachments-editor.html example
- unit testing for low level attachmentsStore.js and attachment funcionality in offlineFeaturesManager.js

things not included (yet)
- handle some corner cases (user deletes an offline feature that has attachments)
- check for hasAttachments attribute to allow/disallow attachment handling
- support for deleting attachments
- support for viewing local attachments
- update README.md
- improve attachments-editor.html to report status of attachments storage