Update exchange rates in background daemon process

This is the only way (I know of) to not block the currency.py script from blocking
when called by Alfred. This should allow the workflow to function perfectly
when Internet connectivity is bad/non-existent.
Rename Help.html to README.html

Makes exporting from Marked easier for me :)
Move non-workflow files out of src/ directory
Open README.html, not Help.html
Add info about supported units.

More info on supported keywords