luigi.s3 api changes.

* Add support for flags other than _SUCCESS.
* Introduce S3FlagTarget instead of S3EmrTarget, as there's really
  nothing EMR specific about that target (which could be confusing).
Ensures that dependencies have at least as high priority as dependents
Updates test/requirements.txt to allow running of all unit tests
Merge pull request #447 from Houzz/update_requirements

Updates test/requirements.txt to allow running of all unit tests
Allows sending of html e-mail
Merge pull request #448 from Houzz/html_email

Allows sending of html e-mail
Removes duplicate loops over deps in priority update
Merge pull request #444 from Houzz/recursive_priority

Ensures that dependencies have at least as high priority as dependents
Move creating dummy tasks for task deps from _update_prority to _add_task

See #444
Merge pull request #428 from jcrobak/s3-api-changes

luigi.s3 api changes.
Merge pull request #452 from spotify/erikbern/444-patch

Move creating dummy tasks for task deps from _update_prority to _add_task
Adds resource limitations for tasks
Add support for adding egg-info files to packages archive.

In order for packages that make use of entry points to work on Hadoop
node, the relevant egg-info directories also need to be copied to those
nodes. This change performs a simple check to see if a related egg-info
is present for a requested package, and adds those files as well.
Changes resource example
Treats empty package.__path__ like it's missing
Add test data for creating package archives with egg-info.
Merge pull request #453 from Houzz/ignore_empty_package_path

Treats empty package.__path__ like it's missing
_hdfs_test: 2.6 backport for assertRegexpMatches
Fix deprecation warning

We should use raise_if_exists over fail_if_exists. I guess my clever and
tricky `deprecate_kwarg` method is a bit too meta. :)
_hdfs_test: Fix failing test case

I think this started to fail since the changes I introduced in
spotify/luigi#393.