Remove TensorFlow dependency

Small Documentation changes... 

Assume a compatible flavor of the tensorflow library is
pre-installed. This allows Donkey to be installed on systems
where tensorflow-gpu is installed w.o. having to fiddle with
dependencies.

Since all install instructions in Donkey docs pre-installs
tensorflow already, this is a minor change.

Add instructions to expand filesystem for the Pi

Update the disk image to use tensorflow 1.7 and support pi3b+

# Conflicts:
#	donkeycar/parts/datastore.py

Typo correction

Disable additional shuffling while training

Before this change dataset was shuffled once and
then on every step random sample was returned.
Problem is that this way we do not guarantee that
all samples from dataset will be used.

Added Amazon link for LiPo battery

Fix validation steps computation

Screw location for Pi cam can result in short