Relax library constraints
Get rid of the snap-extras dependency by copying getJSON and writeJSON

snap-extras pulls in a LOT of extra libraries just so that we can
reuse its getJSON and writeJSON functions.  This can cause trouble for
anyone trying to build these examples, as there are times when
snap-extras is lagging behind in its library constraints.  For
example, today heist-0.14.0 which is not allowed by snap-extras-0.9.
The current version also doesn't allow text-1.2 or aeson-0.8.

Rather than fight with build problems due to a too long list of
dependent libraries, just deceouple snap-examples from snap-extras by
copy&pasting the writeJSON, getJSON functions into the Util module.