Allow for setting of the default read/write consistency levels

[douglaslinsmeyer]

It is advantageous, particularly in development environments to allow the Cassandra/Database object have it's default consistency level (currently coded to QUORUM) set to a different level without passing in a parameter on all ->query() method calls.

I propose adding setDefaultWriteConsistency($level) and setDefaultReadConsistency($level) which would be respected by Database::query()

I will create a PR if this meets approval.

[evseevnn-zz]

Will be implemented in version 0.2.0