[API] Improve Handlers (formerly plugins)

Rename plugins API to handlers to better represent their role.
Modify their API to match the new name.
Handlers are now defined in only one module file to speed up loading.
Handlers can now be dynamically injected in the class.
The short notation now has to be called with a leading $ in order to avoid conflicts.
[API] Rename file handler to export and import handlers

These two handlers will allow to import and export to given formats.
Next step is to implement PNG import/export.
[FIX] Add registerHandler notation in README.md

Change 'file' module occurence to new 'export'/'import' modules.
[FIX] ImageSpec wrong test

This testcase had not been changed according to new 'export' API.