Added rating system

- unit-v: users see vote up/down links if they're logged in
- up or down changes if user has already voted up or down
- units_m: checks if user can vote on unit, gets positive vote
percentage and total number of votes and sends back to controller
- rating table added to schema
Fixes to rating system

- fixed function names
- fixed unnecessary call to function
- changed table to use 1, -1
- changed SQL lines according to the table change
Merge pull request #4 from m-mmoocs/Rating

Added rating system