Allow specification of alternate settings dir via BITMESSAGE_HOME

This feature is for Bitmessage/PyBitmessage#434.  It allows you to set an alternate home directory so that you can run multiple instances of bitmessage with different settings.  To use a different settings dir, specify the `BITMESSAGE_HOME` var:

    BITMESSAGE_HOME=~/.mailing_list_server python src/bitmessagemain.py

@atheros1 note that I put the code in lookupAppdataFolder in shared.py rather than loadConfig(), because we need to know where the new keys.dat file is *before* we load the config.