Generate insertId if not provided.

[stephenplusplus]

Fixes #1041

[coveralls]

Coverage remained the same at 100.0% when pulling 2e44af4 on stephenplusplus:spp--1041 into f10f61f on GoogleCloudPlatform:master.

[callmehiphop]

@tswast based on some dialog within the attached issue, it seems like a uuid per row might not be the way to go. Would you mind taking a peak at this?

[tswast]

UUID-per-row is the way to go (if the user does not provide one).

After some discussion with the BigQuery team, we determined that a row ID should always be included in the request. If not present, create a unique ID for the row through a suitably random source.

The reason behind this is so that the API request can safely be retried by the client library.

[callmehiphop]

Awesome, thanks for the feedback!