[RUBY-32] improve exception hierarchy and documentation
Merge pull request #26 from datastax/RUBY-32

RUBY-32
[RUBY-39] prevent usage of remote hosts by default
increase default heartbeat interval to 30 seconds
skip ignored hosts when waiting for schema agreement
[tests] demonstrate that remote hosts are not used
[ci skip] update CHANGELOG
Merge pull request #27 from datastax/RUBY-39

change defaults
[RUBY-12] rename Cassandra.connect to Cassandra.cluster
Merge pull request #28 from datastax/RUBY-12

[RUBY-12] rename Cassandra.connect to Cassandra.cluster
fix undefined variables
replace hard-coded value with constant