Get sessions host

Issue #11
Ensures sponsor_session has only one host per session

Issue #11
Fixes typo and split long sentence to block

Issue #11
Moves and splits sessions host query to sponsor_sessions scopes

Issue #11
Adds avatar to sponsor

Issue #11
update email template
fix for rake task
update reminder template
Merge pull request #20 from codebar/update-session-emails-2

Update session emails 2
Merge pull request #19 from pvcarrera/session-host

Session host