Allow "options.acceptFileTypes" to be a string

Hi,

I'm implementing the uploader in my AngularJS app and came into tiny problem where I define all the uploader's options in the template:

```html
<div file-upload="{
    'method' : 'put',
    'url' : '/user/avatar/',
    'autoUpload' : true,
    'maxFileSize' : 5242880,
    'acceptFileTypes' : /(\.|\/)(gif|jpe?g|png)$/i
}">
....
</div>
```

Obviously, the ```acceptFileTypes``` option defined like this is not a valid ```RegExp``` object - Angular actually throws an error:

```
Error: [$parse:lexerr] Lexer Error: Unexpected next character  at columns 313-313 [\] in expression [{
    'method' : 'put',
    'url' : '/user/avatar/',
    'autoUpload' : true,
    'maxFileSize' : 5242880,
    'acceptFileTypes' : /(\.|\/)(gif|jpe?g|png)$/i
}].
```

I know that I could define the options hash in the controller (like the demo does), but I wanted to keep the controller as small as possible.

Therefore I've made a tiny tweak that allows the ```acceptFileTypes``` option to be a string that will get converted into ```RegExp``` object (if it's not empty), so then I can use it like:

```html
<div file-upload="{
   ...
    'acceptFileTypes' : '(\.|\/)(gif|jpe?g|png)$'
}">
....
</div>
```