Saner base_dir defaults 

With this pull request the following is now the way base_dir is set:

If no other options are specified and bluepill is run as root base_dir defaults to /var/run/bluepill otherwise it defaults to ~/.bluepill.

If environment variable BLUEPILL_BASE_DIR is set it uses that. This is so you can do the following:
```
export BLUEPILL_BASE_DIR=/var/run/muhpill
bluepill load example1.pill
bluepill load example2.pill
```

Otherwise load now respects the base_dir specified on the command line and uses that over the environment variable fixing #144. This one not being the case always puzzled me. 

But ```:base_dir``` option specified as part of the DSL still takes precedent over all these defaults. 

Oh and I made tests for all these changes.