Allow loading custom css on views

Jira: WP-1234
Fix crash from /invite
If not logged in via facebook or twitter, /invite crashes pzh

Jira: WP-1235
Allow fb and twitter callbackURL to be empty
and fill it automatically with: https://<pzhAddress>/auth/<provider>/return

Jira: WP-1236
Fix pzh crash if fb friend list fails to populate

Jira: WP-1237
Keep authority's image when registering a new pzh

Jira: WP-1238
Instruction for setting up facebook login

Jira: WP-1239
Simplify cross pzh connection

Jira: WP-1212
Allow facebook profile pictures to be loaded

Jira: WP-1240
Typo fix

Jira: WP-1236
Remove unused css

Jira: WP-1212
Merge pull request #30 from cbotsikas/fixesBeforeVersion

Fix Facebook login method & simplify the invitation page + social buttons