Refactors all themes to use UDL2.1. Adds a new Obsidian Theme

Hello !

I've been using your User-Defined Language file for many months now, and I'm glad you published it !
I noticed several shortcomings that really annoyed me, such as bold sentences only having the first word colored.
So I tried to improve it, and I'm glad to say that this pull request is an improvement.

I used the new UDL2.1 (see [Notepad++ Doc][udl2.0]) and used **delimiters** instead of **keywords**. I also noted that multiple syntax did not overlap well, such as _**bold text** in italic_. It now is colored properly, there are still a few minor shortcomings however.

To summarize changes: 

* Spaces between words now only appear in the single asterisk (*) syntax.
* Links can be single words.
* Most of the combinations of delimiters (bold text in italic text, etc) are now displayed properly.
* Escapes slashes (`\`) can now be used. Unfortunately, they only work like string escapes : `\* this would be italic text with an \* asterisk in it *`.

I've updated all three original themes (default, twilight and zenburn) and also added my own theme : obsidian.
Obsidian uses a [color theme from Kuler][kuler-firefox-theme] named "Firefox".

Here's the Obsidian theme : 
![screenshot](https://f.cloud.github.com/assets/2051507/671440/3e3209f4-d85c-11e2-97fa-6f8a3815a89e.png)

I've also taken the liberty of changing the colors in the **default** white theme. I thought that the colors for this theme didn't really "work" together, they somehow lacked a sense of "coherence". So I created a Kuler, I used an "Analogous" rule and so at least the luminosity is similar. See [the Kuler here][kuler-default-theme].

Here's what it looks like : 
![screenshot](https://f.cloud.github.com/assets/2051507/671439/2a4f124c-d85c-11e2-802e-47e671ecb881.png)

Also please note that I **did not** change the colors in Zenburn or Twilight. They do use the new syntax though, so they will work the same as the Default and Obsidian themes.

I've also added a new `syntax_highlighter_test.html` that features many combinations of syntax (bold in italic, code in bold, etc.). It uses the brilliant [strapdownjs][strapdown] library, so if displayed in a web browser it will convert all Markdown text inside to HTML so that one can see the highlighting in Notepad++ and then see how it _actually_ renders in a browser.
Finally, I've added a `syntax_cheatsheet.md` file, copy-paste of the [syntax cheatsheet in Dingus][dingus]. It features most (if not all ?) the syntax in Markdown and should help test further the highlighting.

Thank you for your time, sir !

[udl2.0]: http://udl20.weebly.com/
[kuler-default-theme]: https://kuler.adobe.com/Markdown-Notepad-Highlighter---Default-Theme-color-theme-2586886/
[kuler-firefox-theme]: https://kuler.adobe.com/Firefox-color-theme-629495/
[strapdown]: http://strapdownjs.com/
[dingus]: http://daringfireball.net/projects/markdown/dingus