converted code base to python 3 using 2to3:
 * relative imports
 * moved modules
 * exceptions as syntax
 * print function syntax
 * xrange -> range
 * convert to list dict key and items

basestring & unicode were untouched, needs special handling
some manual fixes where applied
added py3k compatibility layer module (imports and unicode/basestring support)
renamed html test due py3k naming conflict

--HG--
rename : tests/html.py => tests/html_.py
fixed template for py3k
stripped basic invoice test from template module, moved it in tests

--HG--
rename : fpdf/template.py => tests/invoice.py
removed import * (py2.5)
removed as exception to support for py2.5
fixed invoice test to open the pdf on windows
fixed png image handling in py3k (warning with bytes vs unicode and integer division)
re-introduced png in template test
added image to the py3k test
adjust some tests to work on python 3