Setting to enable/disable modeltranslation support

[Sergey Vishnikin]

Using modeltranslation doesn't mean that it will be used for translation of AutoSlugField() I think that autoslug should check not only for presence of 'modeltranslation' in INSTALLED_APPS, but also some setting like 'AUTOSLUG_MODELTRANSLATION_ENABLE'

Please, see this bug for example

[Vadim Iskuchekov]

Try to fix it, like Sergey Vishnikin suggested. Please, check my pull request.

[Andy]

Thank you guys, will be published in the next version.