Adds soundcloud regex to embedda

# Adds Soundcloud
The spec is updated to reflect the new functionallity

## Limitation
Currently to soundcloud feature doens't support
````html
<a href="soundcloud-track-url"> Link to awesome track </a>
````

### Issues
I hit some weird issues when implementing the regex the same way as the existing features, created a question on stackoverflow for it.
http://stackoverflow.com/questions/15913567/escaping-matched-result-from-regex