[2.0][DDC-284] Fixed. API polish and some convention over configuration simplifications for join columns and join tables.

[2.0] Refactored pieces of QueryBuilder, it does not use string concatenation anymore. Splitted from into from and join, allowing update and delete DQLs to be correctly built. Added missing entry set. Removed Expr\Having because it was useless. Updated docs.

[2.0] Fixed issue with functions accepting subqueries. Implemented missing Expr\Join class. Added Expr::*join helper functions.