Skeleton Miranda signals extensions

Not much to this yet, pull requesting to discuss repo format and location of extensions.  Things I did:

* Move Miranda into a "core" folder
* Add "extensions" folder
* Add parent and extensions parent pom files
* Add skeleton signals extension

Specifically I was thinking the repo could look like:

* core
  * Assets
* extensions
  * signals
    * Assets
  * ...
  * pom.xml
* pom.xml