use dir="auto" to have better RTL support for messages

This works pretty well in latest versions of Chrom[e|ium] and Firefox as it's visible in the attached screenshots:

A conversation in Persian **with** this patch:
![with_patch](https://cloud.githubusercontent.com/assets/194719/5424039/99ffa628-82de-11e4-8f71-5e8ff7413852.png)

A conversation in Persian **without** this patch:
![without_patch](https://cloud.githubusercontent.com/assets/194719/5424040/9a005988-82de-11e4-858a-5ee23e1aa90c.png)

related: #182