Clarify contributing guidelines
Update dependencies
Correct Cassandra version dependency in gemspec
Handle both Cassandra 1.2 and Cassandra 2.0 internal names for timestamp type
Fail fast when attempting to change a column's type in an incompatible way

Later patchlevels of Cassandra 1.2 and 2.0 only allow a small subset of all
possible type transitions; fail fast when attempting one that's not allowed.
Fail when a schema migration would change the clustering order
Merge pull request #59 from outoftime/1-2-13-compatibility

Compatibility with Cassandra 1.2.13