EmBlocks EIX exporter support.

Hi,

I have added EmBlocks exporter support which can be used by the special plugin in the next EmBlocks release. I have added some test cases to the export_test.py and have already added the exporter to the export/_init_.py file.

Perhaps also an icon is needed for the front-end?

Additional info at the end of:
https://mbed.org/users/dreschpe/notebook/offline-nucleo-development-with-emblocks/