Set stacklevel to 2 in Derived and Copy helpers.

Currently, the warning messages are not helpful.

For example, without the patch applied the output is:

/home/berker/projects/luigi/luigi/util.py:209: DeprecationWarning: Derived is deprecated, please use the @inherits decorator instead
  warnings.warn('Derived is deprecated, please use the @inherits decorator instead', DeprecationWarning)

With patch applied:

t.py:9: DeprecationWarning: Derived is deprecated, please use the @inherits decorator instead
  class MyTask(luigi.util.Derived(AnotherTask)):
Improve CLI example.

It would be good to be able to just copy/paste and run the example
locally.

This commit also improves the markup a bit.
Remove luigi.reset and luigi.expose methods

I got motivated to do this when I saw some test starting to fail as of
spotify/luigi#530. However, on its own, this is just a cleanup commit.
Merge pull request #560 from Tarrasch/remove_luigi_dot_expose

Remove ancient luigi.expose{,_main} methods
Merge pull request #553 from berkerpeksag/improve-cli-example

Improve CLI example.
Merge pull request #550 from berkerpeksag/fix-stacklevel

Set stacklevel to 2 in Derived and Copy helpers.
Silence "outputs has no custom" warnings in the test suite.