JS Configurablity Take 2

Change the way configurability works.
Config dict should be passed down to the parent class where it will be
merged with the default value and propagate to this only in the base
class.

This allow to both alter the configuration on a per instance basis, or
globaly by tempering with the class instance.

This also get rid of IPython global in some cases.
Merge pull request #2979 from Carreau/cm-configurable

CM configurable Take 2

Change the way configurability works.
Config dict should be passed down to the parent class where it will be
merged with the default value and propagate to this only in the base
class.

This allow to both alter the configuration on a per instance basis, or
globaly by tempering with the class instance.

This also get rid of IPython global in some cases.

-- 

Still not **perfect** but I think this is the limit of my js knowledge, there is a minimal amount of code of 4 line to propagate the configuration :

```
var options = {foo:bar}; // default options can be class parameter
var overwrite_options ={boo:baz}; // came from args, or not
options = this.mergeopt(CodeCell, options, overwrite options);
IPython.Cell.apply(this,[options]);
```