Add switch to configure database SSL connection

PostgreSQL supports secure connections through SSL. For PuppetDB
to connect with SSL, "?ssl=true" has to be specified on the
connection string.

This patch adds such a switch, by default PuppetDB will not use SSL
to connect to the database.
Disable database validation, if SSL is enabled
Document new database_ssl option in README
Merge pull request #80 from stdietrich/add_ssl_switch

Add switch to configure database SSL connection