add basic handling for transient errors

[subvertallchris]

There are at least four statuses that Neo says you should just retry, so that's what we'll do. The number may be a little light in some environments, so it is controllable using the NEO4J_RETRY_COUNT environment variable.

In my staging environment at work, this catches most of our errors. The remaining are going to be handled at an app level.

[coveralls]

Coverage increased (+2.18%) to 94.48% when pulling ff926b6 on retryable_errors into 08499c2 on master.