fix a foreach stmt type checking - related to #12
fix spec files
fix url and source in specfiles
desc compilation on windows
Update README.md

info for compilation on OS X
reorganize tests due changes in plpgsql in 9.4 and 9.5
allow build on OS X
Use PGXS by default.

But provide NO_PGXS for those who want to copy the source to the PostgreSQL
contrib directory and compile like a contrib extension.