Adding auto-retry mechanism when connnection fails to cassandra.

  NOTE: This will retry 10x before giving up.

  Credit to @pezra also for pairing.
Added :max_retries configuration parameter for customization of maximum retries that will be made to reconnect to cassandra

  * Added spec stubs
  * Modified template for config file (added max-retries)
Adding specs for max retries in Cequel::Metal::Keyspace#execute
Changing default max-retries to 3
Defining clearer spec for failed connection testing in Cequel::Metal::Keyspace
Changing default max-retries to 3 in `cequel.yml` template and documentation
Added if_not_exists as an option

Adds IF NOT EXISTS to queries
move if_not_exists option handling to Inserter class
remove unnecessary comment from writer
remove if_not_exists from writer
Changed `max-retries` to `max_retries` and using ` clear_active_connections!` instead of directly accessing ivar
removed cql query string check
changed `attributes` to `options`
Removing unnecessary exception variable and quotes around symbol
Revert "Added if_not_exists as an option"