Sandbox mode support: read the zip contents.

Sandbox mode support: read the zip contents.

This is another incremental step for
the local charm functionality in sandbox mode.

Read the contents of the files (included in the zip)
we are interested in.

Now the only missing bits are updating db.charms and 
returning the expected charm URL to the caller.

QA:

1) make debug;

2) Open the GUI and the js console.

3) Drag a zip containing a charm and click "upload":
    - you should see the 
      "charm upload in sandbox mode is not yet implemented" 
      notification;
    - in the js console, you should also see a list of 
      the three files we are interested in, included in the zip,
      and their contents;

4) Now drag a zip not including a charm: 
   you should see an error notification similar 
   to "unable to find the following required files: ...".