Implement account elsewhere deletion.

First step towards fixing #639.
Add UI to elsewhere account deletion.
Add tests, fix bug.
Merge branch 'master' into disconnect-639

Conflicts:
	gittip/elsewhere/__init__.py
	gittip/models/_mixin_elsewhere.py
	templates/connected-accounts.html
	tests/test_elsewhere.py
	tests/test_participant.py
style change: use an actual button
Merge branch 'master' into disconnect-639
fix error handling
fix error messages
fix tests
Merge branch 'master' into disconnect-639
log account disconnect
rename `elsewhere.json.spt` to `delete-elsewhere.json.spt`
Merge branch 'master' into disconnect-639
Merge pull request #1976 from gittip/disconnect-639

Add ability to disconnect elsewhere accounts.