Don't mistake unordered lists for italic text

Currently, it's possible that unordered lists are confused for italic text. For example, this situation:

![screen shot 2014-05-02 at 2 13 26 pm](https://cloud.githubusercontent.com/assets/38924/2862504/fc683f92-d1f3-11e3-9ab8-06e8434e62c6.png)

should actually be parsed like this:

![screen shot 2014-05-02 at 2 12 32 pm](https://cloud.githubusercontent.com/assets/38924/2862505/0149bcb6-d1f4-11e3-8c5f-bc7b2a1203c4.png)

This PR massages the regex used to detect italic text formatted with `*`, and splits it into two cases to handle the `^* foo*` situation correctly.

cc @shawnyeager (thanks again for the report!)