Add infrastructure to lazily define variable via autoimport

[Carreau]

This should cover the feature request on IPython to have the ability to lazily define variable that can take some time to be created when first access in IPython;

It does so by utilizing the autoimport machinery of pyflyby and creating runtime modules via a import hook.

This has many advantage:

• we do not use a proxy object.
• we do not have to deal with multiple variable being potentially defined together and thik of re-excution.

[Carreau]

One thing to decide is what the name of the high level API will be.

Testing and documentation are missing as well,.

[Carreau]

Ok, it's been two weeks, i'm going to merge that and we can iterate later if needed.