libpixyusb fixes

ITEMS COMPLETE:

+ Protect pixy_command calls when libpixyusb is uninitialized
  This fixes segmentation fault in 'hello_pixy' example when
  pixy is not connected.
+ Remove linefeed (^M) characters from build scripts:
  - host/buildpixymon.sh
  - tools/build_pixymon_src.sh
  This fixes error when calling the scripts from bash.
+ Make white balance disabled by default in 'hello_pixy' libpixyusb example.

TODO:

+ Make 'hello_pixy' libpixyusb example stand alone.