Implove GridFieldAddExistingAutocompleter->doSearch() html output

- Replace HTML entities in GridFieldAddExistingAutocompleter->doSearch()  with actual charaters as the DropDown does not play nicely with HTML entities
-  disable source_file_comments in GridFieldAddExistingAutocompleter->doSearch() because they would be displayed as plaintext in the dropdown

this is what it looks like before this change:
![before](https://f.cloud.github.com/assets/186158/1499001/a2a81c34-484b-11e3-9ab7-63f70f0b3c01.png)

after first commit with source_file_comments on:
![before2](https://f.cloud.github.com/assets/186158/1499158/cf4fd2c2-4850-11e3-9f16-05a5c8128401.png)

and after:
![after](https://f.cloud.github.com/assets/186158/1499002/a752c41e-484b-11e3-8c8b-0213d38a326d.png)