change structure of folders and files to accomodate issue #59

[santoshphilip]

change structure of folders and files to accomodate issue #59

possible strategy (evaluate before implementing):

• create a folder hvachelpers and move hvac.py into it.
• rename hvac.py as exhaust.py - it has only exhaust fan functions now
• replace_hvac.py may be a good name for the file for issue #59