Use our own credential store (#29)

Credentials class which saves credentials to XDG_CONFIG_HOME/p/credentials.json
instead of PyPump's default location XDG_CONFIG_HOME/PyPump/credentials.json

Note: Users upgrading to this version should probably delete
XDG_CONFIG_HOME/PyPump/credentials.json (or move it to the new location)
Dont use pump object in p_accounts

This let's us use p_accounts without having an account activated and/or
authorized