Class label colours with parallel coordinates
#### Problem description
The parallel coordinates function seems to currently assume that the class labels don't have an inherent ordering. I'd like to provide a sort option or a class-to-colour mapping to capture this. My specific use case is situations where the label are quantized buckets and a smooth colour change between buckets make the final plot easy to visualize.

The current function zips the available classes and the colour_values without any sorting.

I've identified where in the current '_parallel_coordinates' function a change should be made and would like to work on this.

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.4.2.final.0
python-bits: 64
OS: Linux
OS-release: 3.2.45-0.6.wd.865.49.315.metal1.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.2
nose: None
pip: 9.0.1
setuptools: 34.3.2
Cython: None
numpy: 1.12.1
scipy: 0.19.0
statsmodels: None
xarray: None
IPython: 5.3.0
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 2.0.0
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999999999
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.5
boto: None
pandas_datareader: None

</details>