add "Brackets" to user agent string, and move remote debugging port to config.h

Fix for adobe/brackets#2213

Modifies the user agent string to include "Brackets". The user agent string now looks like:

```
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_8_2) AppleWebKit/537.1 (KHTML, like Gecko) Brackets Safari/537.1
```

Ultimately, we probably want to add the build number to the user agent string as well (so it looks something like ```... Brackets/0.18.0 ...```). However, this will require some changes to the build process because we'll need to figure out a way to have the build number available at compile time. This is discussed in more detail in issue adobe/brackets#2442.

Also moves the hard-coded remote debugging port to a #define in config.h. By having it in config.h, we can set separate ports for Brackets and Edge Code. This means that we can debug both of them at the same time without having to worry about which one we opened first.