Use assertInstanceOf

Adding YearMonth function (beberlei#266)

Adding MySql YEARMONTH Function

Add EXP, LOG and LOG2 for MySQL

Setup dist to precise

Add STRING_AGG Postgresql function

SUBSTRING_INDEX accepts non-strings

Merge pull request beberlei#249 from vyk12/fix-arithmetic-primary-res…

…triction

Change ArithmeticPrimary to ArithmeticExpression for LEAST and GREATEST functions

Merge pull request beberlei#246 from mkraemer/utc_timestamp

Add support for UTC_TIMESTAMP on mysql

Merge pull request beberlei#234 from bubnovKelnik/master

Add Query/Sqlite/IfElse with tests

Merge pull request beberlei#228 from maxolasersquad/master

Centralize and simplify query test case superclasses.