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
Add support for dynamic requiremnets

Tasks may now yield new requirements in run just as they are returned
in requires. If requirements are fulfilled when yielded the task will
continue, if not it will be rescheduled to be run when requirements are
finished. Note that this means that the task will be rerun from start.
Add dynamic requirement example
Add pyparsing to .travis.yml
Make it compatible with Python 2.6
Make parameter.serialize handle is_list
Change init proceduce for dynamic requirements

It used to be initialized using task_id now it's done using
task_family + str_params.
Add load_task function in interface
Remove id_to_parsable from task.py
Add test for task to str params conversion