Fixed markdown pages in gh-pages

Hi,

I'm newbie of Gitblit.
Thank you for the wonderful software!

I found 3 issues regarding markdown pages in gh-pages in the servlet class  "com.gitblit.PagesServlet".

* Can't provide index page other than "index.html"

Please see the line 152 of the servlet.

```java
content = JGitUtils.getStringContent(r, tree, file, encodings)
							.getBytes(Constants.ENCODING);
```
If the "file" doesn't exist, JGitUtils#getStringContent() returns null, and this causes NPE.
So the servlet can't provide other than the first file "index.html".

* Can't provide markdown files with "text/html" content type

Currently, content type is determined in the code 165-168.

```java
					String contentType = context.getMimeType(resource);
					if (contentType == null) {
						contentType = "text/plain";
					}
```
In the case of "mkd", "md", "markdown" and so on, ServletContext#getMimeType() returns null then markdown files is provided with "text/plain" content type.

* Only  index.mkd is supported as default index page [ This is improvement ]

Default index page files were constants (index.html, index.htm, index.mkd).
User defined markdown extensions should be contained in these files.