Added pessimistic locking option to queries
Added lock to list of query expression keywords
Updated documentation for the lock option
Added test to check that produced SQL includes FOR UPDATE
Added keyword lock to the README
Allow passing a database-specific locking clause as a string value to
the lock query expression
Added integration test to verify SELECT ... FOR UPDATE
Replaced use of String.valid? with is_binary
Use the existing "posts" table in the lock integration test
Add comments to the lock integration test
Merge pull request #178 from pma/master

Added pessimistic locking option to queries