Fixed SF bug #1297

This changeset resolves several issues related to python imports.
For example, it's possible now to import modules having same module
names, but belonging to different packages.

From the user's viewpoint, this patch gives more control on import
directives generated by swig. The user may choose to use relative or
absolute imports -- the new python3 syntax for explicit relative imports
is now available as an option.

Some details:
  - we (still) generate import directives in form 'import a.b.c' which
    corresponds to absolute imports in python3 and (the only available)
    ambiguous one in python2.
  - added -relativeimport option to use explicit relative import syntax
    (python3),
  - added -importininit option to enable some workarounds allowing
    to import sub-modules from within __init__.py files (works only with
    -relativeimport)

Tests are under Examples/python, these are in fact regression tests but
with the current swig testing framework it seems to be impossible to put
appropriate tests under test-suite.