Support for the PIC18F25K50, PIC18F45K50, and the P-Star 25K50 Micro.

As we discussed earlier, this pull request adds support for the PIC18F25K50, PIC18F45K50, and our [P-Star 25K50 Micro](http://www.pololu.com/product/3150) (a breakout board for the PIC18F25K50).

I tested all of the apps on the P-Star, and I also ran the unit tests.  The only problem I noticed was that when I loaded the hid_composite app onto the P-Star and connected it to my Windows 8 64-bit computer, I saw a "USB Input Device" in the Device Manager with a yellow warning sign and when I double-clicked on it, it said "This device cannot start. (Code 10)".  The problem went away when I told Windows to "Uninstall" the device, so it was able to forget that the device was single HID (which it learned while running the hid_mouse app) and correctly learn that the device is now a composite HID.  That type of problem should be fixable by using different product IDs for the different types of USB interfaces presented in the examples.

Thanks!