Change delegate methods to be Swift compatible and adhere to Cocoa Guide...

Hello,

This is a mostly mechanical diff to make GMGridView compatible with Swift while using Objective-C Bridging.

Basically, delegate method names beginning in 
```
- ($TYPE)GMGridView:(GMGridView *)gridView ...
```
 collide with the class name. Also, as per Cocoa Guidelines, I stripped the GM prefix from method names, so the new methods look like this

```
- ($TYPE)gridView:(GMGridView *)gridView ...
```

ciao

Luca