Fixed #1709 and added information for not supported old browsers
create platform -> class registry for AccountElsewhere
leave registry interface as dictionary
wip refactor
add root exceptions module and move participant exceptions there
remove username helpers comment
Merge pull request #1849 from venmo/move-exceptions-out

Improve exceptions organization
added suggestions to code
Merge pull request #1829 from Solution4Future/old_browsers

Fixed #1709 and added information for not supported old browsers
Merge branch 'D-refactor-platforms' into link-venmo
Merge branch 'refactor-platforms' into link-venmo

Conflicts:
	gittip/elsewhere/__init__.py
Merge branch 'master' into link-venmo

Conflicts:
	gittip/elsewhere/__init__.py
	gittip/models/participant.py
	www/%username/username.json.spt