Fix #571: Footer broken fix

Fix #571 

# Introduction: #

This problem has been asked so many times in readthedoc's issue section, still no easy fix.
 - readthedocs uses its own layout.html, so custom layout.html will be ignored (https://github.com/rtfd/readthedocs.org/issues/545)
 - some other attempts on fix this problem(https://github.com/rtfd/readthedocs.org/issues/152)

What's worse, the footer is no longer inside \<div class="footer"\> in sphinx_rtd_theme. It's inside some other div, so even hard to make changes.

# My Procedure: #
1. Grab the source code from [sphinx_rtd_theme](https://github.com/snide/sphinx_rtd_theme)
2. Navigate to sphinx_rtd_theme folder
3. Modify the last section from footer.html:

    ```
    {% trans %}<a href="https://github.com/snide/sphinx_rtd_theme">Sphinx theme</a> provided by <a href="https://readthedocs.org">Read the Docs</a>{% endtrans %}
    ```
    - to the following:
    ```
    {% trans %}<div class="footer">
  © 2013 FOSSLC
  | <a href="/contact.html">Contact us</a>
  | <a href="/sitemap.html">Sitemap</a>
  | <a href="https://github.com/Freeseer/freeseer/issues/new" target="_blank">Report an issue</a>
  </div>{% endtrans %}
    ```
4. Now, Copy the whole sphinx_rtd_theme folder(sphinx_rtd_theme source one) to our docs:

    ```
    cp ./sphinx_rtd_theme/* ../freeseer/docs/source/_themes/sphinx_rtd_theme/*
    ```
5. Then, modify the conf,py in our project to use the custom theme under _themes directory

    - Symlink or subtree the ``sphinx_rtd_theme/sphinx_rtd_theme`` repository into your documentation at
``docs/_themes/sphinx_rtd_theme`` then add the following two settings to your Sphinx
conf.py file:

    ``` python
    html_theme = "sphinx_rtd_theme"
    html_theme_path = ["_themes", ]
    ```

# Result: #

After completing above steps, I tested it under my readthedocs test account.

It works!!! http://freeseer-test-umcsdon.readthedocs.org/en/latest/

Note: sphinx_rtd_theme uses MIT license, so we can modify the source code for own use