Allow starting pisignage from a different directory than where it was checked out

[raboof]

Currently, pisignage-server expects to be started from the directory where it was checked out / installed: for example, it load the views and public directories relative to the CWD.

I'd propose:

• view should be immutable and relative to the location of server.js
• public should be immutable and relative to the location of server.js
• config/env/all.js should be immutable and moved to config/defaults.js relative to the location of server.js
• config/env should be configurable, default to $CWD/config/env
• data should be configurable, default to $CWD/data
• media should be configurable, default to ../media relative to the location of server.js

If this makes sense I'd be interested in contributing this feature. WDYT?