Log Message:
Update to 0.3.7

Changelog:
15/11/2015
- RagTime v5-v6 parser: try to retrieve the embedded spreadsheets
  + begin to understand the hierarchical structures which store
    a chart (but not the meaning of each field),
- ClarisWorks/AppleWorks: find a new structure in the file's header
  (which could prevent to convert some database's file),
- BeagleWorks/GreatWorks/Word Perfect Works: retrieve the picture's
  field in a database
- all: improve a little the methods which create spreadsheets,
- all: make some constructors explicit.

10/13/2015
- add a parser for Style documents(at least v1.6 and v1.9)

10/9/2015
- add file signatures for FullPaint documents which have the same format as \ 
MacPaint..

10/8/2015
- add a parser for Pixel Paint documents