Add Pseudo Mode to server configuration

GerritServer needs to be enabled in order to use it at several sections.
It needs actual gerrit connection. But it is inconvenient to post events
from external.

This patch adds pseudo mode to server configuration. This can start
GerritServer without any gerrit connections.
Update help for pseudo mode
Merge pull request #123 from rinrinne/pseudo-server

Add Pseudo Mode to server configuration