minor improvements, fixed Bug in AP_Inertialnav

overview: (see commit messages for detailed descriptions)
- added consts, comments, defines 
- minor optimizations/improvements 
- added avr-size output to make

- AP_Buffer: 
		rename functions and altered behaviour to conform with STL-container naming conventions 
			- only used within AP_Inertialnav -> changes don't effect other parts of the code 				
- AP_InertialNav: 
		fixed bugs