Add sponsor

Issue #11
Add address

Issue #11
prepare session invitations for next week
Merge pull request #14 from codebar/prepare-next-session-email

prepare session invitations for next week
Add sponsors to sessions

Issue #11
Create many to many relation between sponsor and sessions

Issue #11
Removes cyclic dependence between Address and Sponsor
Merge pull request #15 from pvcarrera/add-sponsors

Creates sponsors/host entities