Documentation: Change doxygen build process

Doxygen build modifications:
    - Update doxygen requirement to 1.8.4
    - Add custom header.html with MADlib keywords and links to previous versions
    - Add custom madlib_extra.css with custom CSS
    - No colons on headers anywhere
    - Remove CSS patch and include that content in custom CSS file
    - Increase font size in TOCs
    - Enable markdown processing
    - add keywords to docs
    - increase toc.li font size
    - remove Files from topics
    - remove diagram
    - define src/images directory
- Reformat various modules to reflect doxygen changes
New Module: Timeseries analysis using ARIMA modeling

Pivotal Tracker: 54248646

Additional authors:
    Shengwen Yang <syang@gopivotal.com>
    Rahul Iyer <riyer@gopivotal.com>

Module includes:
    - Training an ARIMA model for non-seasonal univariate time series
    data
    - Forecasting future values in the time series using trained model
    - Grouping and seasonal data are currently not supported and
    are planned for future work