added optional conversion of value in get_setting()

Example bool:
```xml
<!-- addon_dir/resources/settings.xml -->
<setting id="test" type="bool" /> 

<!-- addon_data/settings.xml -->
<setting id="test" value="true" /> 
```
```python
>>> print repr(plugin.get_setting('test'))
'true'
>>> print repr(plugin.get_setting('test', bool))
True
```

Example enum:
```xml
<!-- addon_dir/resources/language/English/strings.xml -->
<string id="30110">Resolution</string>  
<string id="30111">480p</string>
<string id="30112">720p</string>
<string id="30113">1080p</string>

<!-- addon_dir/resources/settings.xml -->
<setting id="resolution" type="enum" label="30110" default="1" lvalues="30111|30112|30113"/>

<!-- addon_data/settings.xml -->
<setting id="resolution" value="2" />
```
```python
>>> print repr(plugin.get_setting('resolution'))
'2'
>>> resolutions = ('480',  '720', '1080')
>>> print repr(plugin.get_setting('resolution', choices=resolutions))
'1080'
```

Of course the same with int and unicode.

regards,
sphere