Adds explicit CRC width and PA level. Changes hardware initialization

order to minimize noisy/false message reception. Removed begin call
from constructor as begin now has delay which creates timer race
initialization. The delay exists to allow for power on/reset radio
settling which can otherwise prevent CONFIG bits from holding. Added a
method to allow CRC hardware validation to be disabled. This is a
requirement for various promiscuous listening RF applications. Setting
CRC width now forces CRC hardware validation enable.

RF24.h

@@ -506,6 +506,12 @@ public:
      */
     void setCRCLength(const rf24_crclength_e length) const ;
 
+    /**
+     * Disable CRC validation
+     *
+     */
+    void disableCRC( void ) const ;
+
     /**@}*/
 };