buildstep.py: Add support for description{,Done}

Add support for attributes description and descriptionDone in the
BuildStep class. This means that any subclass of BuildStep will also
support these attributes as well. This was not previously the case.
buildstep.py: Remove whitespace damage
test_process_buildstep: Add test for new describe()

Add test for the new functionality of the describe() method in
buildstep.BuildStep.
test_steps_master: Update tests to work with describe()

The altered tests did not work with the new functionality of the
describe() method in buildstep.BuildStep. They explicitly looked for
the name of the BuildStep when querying the status text of a
successful BuildStep, even if said BuildStep had its description-
attributes set. This commit fixes that behavior.
buildstep.py & shell.py: Extract common attributes to superclass

The attributes `description`, `descriptionDone` and
`descriptionSuffix` in the class `ShellCommand` have been moved to its
superclass `BuildStep`. The test for the `describe()` method that uses
these attributes has also been updated.
buildstep.py & shell.py: Moved descriptionSuffix logic

Moved descriptionSuffix logic from shell.ShellCommand to
buildstep.BuildStep
test_process_buildstep: Updated tests for descriptionSuffix
cfg-buildsteps.rst: Move description-attributes to BuildStep

This commit moves the description, descriptionDone, and
descriptionSuffix attributes to the BuildStep part of the document to
reflect the fact that they are moved to the BuildStep class in the
code.
cls-buildsteps.py: Adds info for description{,Done,Suffix}
relnotes/index.rst: Add info about moving description attributes

description, descriptionDone and descriptionSuffix have been moved
from ShellCommand to its superclass BuildStep.
Merge branch 'describe_fix' of git://github.com/eddie-dunn/buildbot