cmake compilation

Hi

If you're interested, I've written some CMakelists and cmake module in order to compile lag && lidarquatree. 
  
  * It has been tested on ubuntu 12.10 and debian testing. 
  * it find lastools automatically if downloaded next to lag
  * in order to not conflict with other liblas It need that lastools/liblas.a must be rename liblaslib.a
  * README.md have been updated with some cmake instruction (not yet complete, lack some image)

Nicolas David