Align td and th with css instead of align property

When you align tables with css, <td align="..."> gets overridden.
Using inline styles keeps alignment no matter what CSS contains.
Add basic assertions for the HTML_TOC render

Since we don't have any tests for the render object, we should ensure
that a minimum set of assertions are passing before trying to improve it
with additional stuff.
Fix Prior to -> Starting with in the README

Thanks @parkr!
Specify the minimum required Ruby version (1.9.2)
Add callbacks from f2ad99f to the README [ci skip]

Add the following callbacks to the README:

  - footnotes(content)
  - footnote_def(content, number)
  - footnote_ref(number)
Allow to set a maximum rendering level for HTML_TOC

Allow the user to pass a `max_level` option when instantiating a new
HTML_TOC render object in order to limit the nesting level in the
generated table of content.
Adding space
Merge pull request #279 from vmg/html-toc-render

Allow to pass a nesting_level option for HTML_TOC
Merge pull request #281 from patoroco/patch-1

Adding space
Merge pull request #147 from julian7/patch-1

Align td and th with css instead of align property
Remove Sundown and Upskirt remains

Remove pre-processor directives about Upskirt and Sundown. The functions
prefixed with "sd_" remain the same not to destroy the blame view on
files.
Merge pull request #282 from vmg/sundown-remains

Remove Sundown and Upskirt remains
Set SD_AUTOLINK_SHORT_DOMAINS flag always on.
Merge pull request #284 from t-yuki/autolink_short_domains

Autolink short domains
Add rule to ignore temporary vim files

I also reordered the wildcard rules in alphabetical order of extension.
Merge pull request #287 from eric-brechemier/gitignore

Add rule to ignore temporary vim files [ci skip]
Add steps needed to configure environment before running unit tests

It took me some time to figure out how to get a properly running environment.
I first had to understand what I needed (e.g. bundler, openssl support)
and then a simple way to install it.

The first issue that I encountered after installing bundler:
http://www.beginnerruby.com/rails-troubleshooting/fixing-opensslbundler-issue-for-rails-on-debian/

And then I had to reinstall ruby... I decided to go with the RVM approach,
which was simple enough as the installation provided feedback at different
steps to indicate how to remove conflicting packages previously installed
in Ubuntu and avoid potential errors.

There may be a need for an additional step to enable openssl support in ruby:
I did not run either `rvm pkg install openssl` before installing ruby with
rvm, neither did I provide the parameter --with-opensll-dir when installing
ruby with rvm `rvm install 1.9.3 --with-opensll-dir=$HOME/.rvm/usr` but I
assume that this was taken care of either by running `rvm install 1.9.3` or
`rvm requirements` which I have not included in these steps because it looked
from the console output like it was run by `rvm install 1.9.3` anyway.

Following discussion with Robin Dupret in the pull request, I improved
formatting and clarified the setup of the environment; I also added the
final steps with the aim of getting contributers to squash all commits
into a single one for each pull request.

I also replaced the link about topic branches; the previous link now
redirects to a generic page:
http://www.youtube.com/GitHubGuides

I replaced it with a link to "Topic Branches" subsection
in section "3.4 Git Branching - Branching Workflows"
in chapter "3. Git Branching" in the "Pro Git" book,
written by Scott Chacon.
Merge pull request #285 from eric-brechemier/contributing

Contributing: Add steps needed to configure environment before running unit tests [ci skip]
Cleanup in the code

Remove extra headers and functions from the code to have cleaner files
since some of them are a bit messy.
Install the tidy command on Travis

Our conformance test didn't run correctly since we hadn't installed the
tidy command. Also update the contribution guideline with the install
command for few operating systems.