[SPARK-5090] [EXAMPLES] The improvement of python converter for hbase

Hi,

Following the discussion in http://apache-spark-developers-list.1001551.n3.nabble.com/python-converter-in-HBaseConverter-scala-spark-examples-td10001.html. I made some modification in three files in package examples:
1. HBaseConverters.scala: the new converter will converts all the records in an hbase results into a single string
2. hbase_input.py: as the value string may contain several records, we can use ast package to convert the string into dict
3. HBaseTest.scala: as the package examples use hbase 0.98.7 the original constructor HTableDescriptor is deprecated. The updation to new constructor is made

Author: GenTang <gen.tang86@gmail.com>

Closes #3920 from GenTang/master and squashes the following commits:

d2153df [GenTang] import JSONObject precisely
4802481 [GenTang] dump the result into a singl String
62df7f0 [GenTang] remove the comment
21de653 [GenTang] return the string in json format
15b1fe3 [GenTang] the modification of comments
5cbbcfc [GenTang] the improvement of pythonconverter
ceb31c5 [GenTang] the modification for adapting updation of hbase
3253b61 [GenTang] the modification accompanying the improvement of pythonconverter