Shows audio/video size limit in "size limit exeeded" toast + increase from 1 MB to 5 MB

This should meet the needs of  #834, #781 and #700.
New translations is needed for the two exceeds_message_size_restrictions strings as the size limit in MB is added as parameter.

![size](https://cloud.githubusercontent.com/assets/5326916/2556545/3b2c5674-b6d7-11e3-97fa-f10dbe94657a.png)


Also i increased the limit from 1 MB to 5 MB. (Screenshot was taken before that)
Telegram allows 1 GB, Viber 10 MB and WhatsApp 16 MB I think.
Though 1 GB is possible according to @moxie0, 5 MB should be fine until the auto download can be turned off or at least set to "wifi only".

Keep in mind that *image* attachments are not limited right now. I just sent a 35 MB file successfully... It took 20 minutes to send+receive at 16 Mbit DSL but it worked.