Unify the way joomla handles alias for articles

#### Scope
Joomla has two different approaches when it comes to automatically handle the aliases of an article. In batch and copy methods will automatically create increments of the alias that has to be replicated, and in creation of a new article will give an error message  ```Save failed with the following error: Another article from this category has the same alias```. This PR aligns the last to the former allowing automatically creation of a known alias (if the alias already exists) and trowing a warning message informing the user for further action (if they want to change it, altogether).
This PR only handles new articles with the field alias left EMPTY
and is a response to the conversation in this #4246

Also I realize the significance of the alias, url slug in SEO, (thats if the site uses the Search Engine Friendly URLs) but:
1. There is a warning, so the user can take further steps to fix it, if he wants 
2. I guess this is a really minor comparing it to the broken routes that Joomla still can so easily create #3415

#### B/C
Although I don’t see how this can be a backwards compatibility break I realize that is a major change in the UX. Instead of denial to save, Joomla will come up with a successful save AND a warning.

#### Testing
Start with a new article, with a title ```something new``` or whatever you want, leave the alias field empty
save it 
repeat the process with the same title, leave the alias field empty, save it
error ```Save failed with the following error: Another article from this category has the same alias``` and no save 
Apply the patch 
create an article with the same title, leave the alias field empty save it.
Success and a warning message
Repeat with Unicode alias enabled in config and try getting title in another language (in multilingual setup)

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Current Joomla behavior:
![screenshot 2014-09-21 19 06 47](https://cloud.githubusercontent.com/assets/3889375/4349216/6a106a88-41ac-11e4-9c87-9afc8c7a4584.png)
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After patch:
![screenshot 2014-09-21 19 06 03](https://cloud.githubusercontent.com/assets/3889375/4349219/7ba08742-41ac-11e4-8b34-0f9c834e8470.png)

 
I would prefer "Alias already existed so a number was added at the end. You can re-edit the article to customise the alias"