Add a REPEAT function. 

Because Postgres has one, and it's part of the JDBC tests. Plus test to
verify functionality.
Update the Repeat function to return an empty string if repeat count is
<= 0. Update testing to verify.
merge tjoneslo: Add a REPEAT function.

Merge pull request #1043 from tjoneslo/sql-layer/add-repeat-function
https://github.com/FoundationDB/sql-layer/pull/1043