First attempt to compile libdb_agg library using cmake
Fixed cmake file that compiles the library:
 - do not include all postgres includes, this will become
   confusing to the compiler
Added package-discovery cmake scripts
Added the main and the library-compilation cmake scripts
Removed explicit dependency on postgres version, removed unneeded logging
Set library name to libdbagg to have unique cmake target names.
Output library name remains libdb_agg
Added db_agg_config target
Added install targets
Bugfix: put includes into include/db_agg directory
Bugfix on include  directory
Merge pull request #15 from zalando/feature/cmake_build

Feature/cmake build