Relative image URL's are not resolved correctly!

My JSON image list contains *relative* image URLs.  These do not display in the imagebrowser window, but if I manage to click on the tiny 2x2 pixel dot standing in for the image, it gets successfully added to CKeditor's image dialog.

The problem is that browser.js needs to know the value of the CKeditor's config.baseHref.  This modification sends it in the querystring.
Added a base tag; the href attribute will be set via javascript if it's required.

(Also changed <tag /> to <tag> in two places as this is HTML, not XHTML)
If the CKeditor instance has config.baseHref set, set <base href="..."> accordingly on the imagebrowser document.

Before doing this, it's necessary to protect the stylesheet URL from being affected.  This is done by setting the link href to an absolute URL (gotten by replacing everything after the last "/" in location.href with "browser.css").

I also made the "thumb" propery optional in the server's JSON objects, in case there are no thumbnails for the images.  If missing or empty, it just defaults to the value of "image".
Minor CSS fix (unrelated to the <base> tag issue).

If you want to see those rounded corners, you need to ensure the image is clipped properly - hence "overflow:hidden".  Otherwise the corners of the image cover the rounded parts of the border.

I also ensured that the thumbnails are scaled down if either dimension is greater than 200px, while still maintaining aspect ratio.  This is particularly useful if you don't use thumbnails at all, just full-size images.

Note that mslinn's CSS update also does this, but not the rounded corners fix.  (Plus he does a lot of other stuff also).

So actually, if implementing mslinn's stylesheet, just add "overflow:hidden" to the .thumbnail class and forget about this update.

Otherwise, this is kind of a minimum-change CSS fix.
Only pass a baseHref query-string parameter if one is defined

Until now the string "undefined" was being passed, which browser.js
later tried to use.
Minor refactoring
Improve the look of the folder switcher

Huge thanks to @mslinn (pull request #1) and @deant (pull request #2)
for their work on an improved UI for the image browser.

This is a "manual cherry-pick" of what @deant proposed in, but:
	- fixes page jumpiness when hovering over folders
	- makes the switcher work for a larger number of folders
	- changes the look slightly
Make the browser not appear stuck when no images are available
Update README
Merge branch 'base-href-fix'
Merge branch 'fancy-folder-switcher'
IE fixes
Improve installation instructions