job_coordinator: fix the return value of retry_task in case of failure

This will fix the error messages when there are many task failures. Now the
erorr message clearly shows that the reason the job failed is because a single
task had many failures.
Merge pull request #572 from pooya/571

job_coordinator: fix the return value of retry_task in case of failure
disco core: avoid creating an instance of worker just for reading blobs
disco core: remove the Params class from disco core

This has been deprecated for a while. This might break some of the clients that
should switch to using the new import location
disco worker: rename ClassicFile to StreamCombiner

The old name did not make much sense.
disco core: remove the classic_iterator alias for result_iterator

The name result_iterator is by no means accurate, but it is widely used by the
the disco jobs. In the future, it should be renamed to something like
disco_iterator.
Merge pull request #573 from pooya/core_cleanup

Core cleanup
ddfs: hash the large blob names to avoid going above 255 characters

This commit should resolve issue #574.
Merge pull request #575 from pooya/574

 ddfs: hash the large blob names to avoid going above 255 characters
travis ci: add erlang 17.1 to the build matrix
examples: add a page rank example
Merge pull request #576 from pooya/page

examples: add a page rank example
examples: add a mapreduce solution for travelling salesman problem
Merge pull request #578 from pooya/tsp

examples: add a mapreduce solution for travelling salesman problem
examples: add ipython example
readme: add ipython screenshot
Merge pull request #581 from pooya/ipython

Ipython
readme: fix the path to the ipython example
readme: add links to disco wiki pages
doc: fix the release date of disco 0.5.3