[New feature]Allow skins in TinyMCE

This will enable TinyMCE to use skins
What problem is being solved
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Right now in Joomla! is hard to customize the look of the tinyMCE editor (needs some core hacks).
This PR will enable the use of skins just by copying them in the respective folder.

B/C
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There shouldn’t be any backward compatibility breaks here.
In the options of tinyMCE instead of having the default as the only option in drop down list, all the available skins will be listed by the repeated names. Also the string that translates to default is useless but I didn’t deleted as I don’t know how language files are treated…
 
How to test
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Need to set editor to tinymce:
1. Apply the patch with patchtester.
2. Edit an article. Everything should be ok.
3. Go to http://skin.tinymce.com and select the preset charcoal or tundra and download it. (lightgray is the one that ships with Joomla!)
![screenshot 2014-09-07 15 10 54](https://cloud.githubusercontent.com/assets/3889375/4178260/a6eda45e-3688-11e4-9a59-17115eda7569.png)
4. Extract the skin in /media/editors/tinymce/skins
5. Go to plugins->tinyMCE and select the skin you just copied.
![screenshot 2014-09-07 15 12 34](https://cloud.githubusercontent.com/assets/3889375/4178262/b0f659e6-3688-11e4-93a3-8382cb0b5922.png)
6. Edit an article with your new skin!
![screenshot 2014-09-07 15 13 21](https://cloud.githubusercontent.com/assets/3889375/4178263/b728b368-3688-11e4-9df6-5e976bedb9bc.png)

Possible incompatibility
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Due to the difference between posix and windows structure of the folders, this PR might need some modifications to work on win machines, but my server set up is on a mac and I don’t really care to set up a win server. So someone with M$ testing environment should test this and report any problems or even provide some code if he/she likes so.

Can you use `defined('_JEXEC') or die;` here?