Add hackSql Invoker/Executor method

for replacing the sql generated by Slick's query compiler by hand-written sql

Slick's type-safe API becomes unusable, when the compiler generates queries the optimizer of the targeted db can't efficiently deal with. A known case are nested subqueries, where MySQL <= 5.5 stops may stop using indices and do unnecessary table scans. This implements a viable workaround, which allows to replace the generated SQL by hand-written SQL.

This is also useful for injecting type-preserving features not supported by Slick such as index hints.

Replaces suggested hack in #807.

review by @szeiger 

Two related mailing list threads:
https://groups.google.com/d/msgid/scalaquery/ee4cdbfe-cf2f-4497-afd2-d27bbc3b8949%40googlegroups.com
https://groups.google.com/d/msg/scalaquery/4O1Wr4EK-W4/b7FCudlia2YJ