Adding "View Source" link to the class page

This is an implementation of #63 task based on original commit made in #64.

___Usage___
By default Sami doesn't know on which hosting Git repository is located and doesn't add `View Source` links on class detail pages. However if you specify `remote_repository` option with instance of `AbstractRemoteRepository` class sub-class, then the `View Source` link would be added:

![sami_viewsourcelink](https://cloud.githubusercontent.com/assets/1277526/3411930/ed431ede-fdf8-11e3-89df-0d6019d5f51c.png)

For example:

```php
array(
...
'remote_repository' => new GitHubRemoteRepository('username/repository', '/path/to/repository'),
...
)
```
The `'/path/to/repository'` is required, because Sami doesn't know base folder on GitHub (or any other service), where original files (for which documentation is being generated) live.

Thanks to @anlutro, which commit (squashed) was used as a base for my changes.

Closes #63
this should be ``null`` rather than ``false``