bug-56: do the followings:
1. add download activity
2. add listadapter for language
3. change wordsstorage to support one db for a lang and category
combination.
4. add dialogutils
5. add errorutils
6. option dialog for partial downloaded db
Merge pull request #15 from john-hu/bug-56-download-ui

bug-56: do the followings, r=thomas.