Retrieve type and variable information from docblock tags

Make the phpDocumentor libraries do the work of splitting up docblock tags into their component parts.

This simple change adds two new pieces to tag entries: types and variable. For example:

```
@param int|object $post Post ID or object.
```

gives

```
{
  "name": "param",
  "content": "Post ID or object.",                                                                            
  "types": [
    "int",
    "object"
  ],
  "variable": "$post"
},
```

Todo: use this information at the higher levels.

Aims to fix #4 and #5.