Spark 1916

The changes could be ported back to 0.9 as well.
Changing in.read to in.readFully to read the whole input stream rather than the first 1020 bytes.
This should ok considering that Flume caps the body size to 32K by default.

Author: David Lemieux <david.lemieux@radialpoint.com>

Closes #865 from lemieud/SPARK-1916 and squashes the following commits:

a265673 [David Lemieux] Updated SparkFlumeEvent to read the whole stream rather than the first X bytes.
(cherry picked from commit 0b769b73fb7ae314325857138a2d3138ed157908)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>