Match issues and be more specific with mentions

This is just a follow up to the Match Mentions PR.

## Update to Match Mentions

```diff
-    'match': '(?<=^|\\s)(@)(\\w+)'
+    'match': '(?<=^|\\s)(@)(\\w+)(\\s|\')'
```

The `(\\s|\')` on the end requires whitespace *or* an apostrophe. This means you can say @bob's like GH allows, but no other special characters. I'm not sure if this scrictness is needed but seemed like a reasonable addition.

## Match Issues

* #9 matches the first issue.
* They can only have numbers so #yolo doesn't match. I wonder if this should be less strict so that issues can also double up as tags...?
* They can be followed by an apostrophe, e.g. "#14's changes".

This all works correctly in the editor:

![untitled_preview_-__users_andytlr_github_language-gfm](https://cloud.githubusercontent.com/assets/475255/2546042/edac7964-b628-11e3-8850-222a326dde4d.png)

Referencing issues breaks the [markdown-preview][1] as they're mistaken for headings. However, that's an issue with [markdown-preview][1].

![untitled_-__users_andytlr_github_language-gfm](https://cloud.githubusercontent.com/assets/475255/2546048/1870dd84-b629-11e3-9109-de5a031f5463.png)

[1]: https://github.com/atom/markdown-preview
[2]: https://github.com/atom/language-gfm