Also adds item symbols definitions

 - it's not a pdf image anymore
 - only the logo (with its text) is an image
 - the color blocs use the beamercolorbox env.
 - Uses the column env. to align the logo

In beamer some environments,  like itemize for example,can only be
called inside a frame.
So the title block is generated in the frame headline and
postertitle command is not needed any more.

 - The theme uses beamerposter package
 - TODO: some refactoring to group common definitions with the beamer
   theme

Reduction  of the font size of the the element shown in the title page
 title
 subtitle
 author
 mail
 date

The split_theme branch is merged
The if@option declaration seems correct now

no background color for the boxes that contain the section/subsection.

Like beamer split outer theme, added a header line with the list of
sections/subsections
TODO: see how we can add options to the theme to have a unique package!
Then merge with the master branch.

(Thanks S.T.)