[FIX]: change correct label and fields in Event Analysis.
Merge pull request #111 from odoo-dev/7.0-bug_1287200-ptr

[FIX] event: change the field nbevent to make sense

The field nbevent was representing the number of registration with value of 1 for events without registrations (which makes no business sense). Fix to actually represent the number of registrations and display a better field name.