* cuda update:
   * now requires cuda 6.5 to compile+run
   * added support for sm_37 (whatever that is), sm_50 (gm10x, first-gen maxwell) and sm_52 (gm20x, second-gen maxwell?)
   * the low-level cuda api now has proper error code -> string functions, use them
   * added a getter to cudacl to get the vector of all cuda devices