fixed a number of problems related to batch lookups.

[poddus]

• batch lookups were limited by the var max_variants_per_batch
  to 200 variants per GET request, which was inconsistent with the
  recommended 10,000 variants per batch, this was corrected in client.py
  along with some small readability changes.
• batchRequestClient.py
  • now uses standard default values implemented in argparse, and takes
    advantage of the "chunking" mechanism already implemented in
    the client batch_lookup function.
  • Note that this breaks the "chunk reporting" functionality of the
    previous implementation, where a notice was printed to STDOUT
    for each chunk. Additional messages were added for the request
    and writing the output file.
  • The option to pass parameters via CLI was added in a way which is
    consistent with run.py.
  • ref_genome was converted to use strings instead of integers,
    to be consistent with documentation.
• run.py was modified for readability and to be consistent
  with batchRequestClient.
• Note that the default values from argparse are separate from those
  found in the client. Calling the variantapi client from
  batchRequestClient.py overrides the default value for
  max_variants_per_batch.
• README was modified to include batchRequestClient.