Made changes in constants.py so that it can automatically create the db directory and set the PERSIST_DIRECTORY enviroment variable if people forget to do it.

[GazPrash]

Made changes so that "db" to store databases (labelled under environ name **PERSIST_DIRECTORY**) directory can be automatically created and be assigned as their environment variable as most people forget to do this quite often and the error they receive is very confusing to follow and doesn't explain the situation at all.

p.s This was my first ever open source contribution so forgive me if this was useless or not needed. Thank You!