Fix for the missing compiler option debug_info

Without the ```debug_info``` compiler option boss_db models can’t be analyzed with dialyzer. See #147 for more information about the error. This PR introduces a new boss_db config option, which let the developer set certain compiler flags like ```debug_info``` (see Erlang docs ```compiler:file/2``` for more options). This config defaults to ```[verbose, return_errors]```, as it’s defined in boss_compiler.erl. Alternatively will ```debug_info``` now be set for models, if it’s defined through erl_opts in the rebar.config.