new: engine rewrite to provide easier writing of new formats.

These are the biggest API changes:

- attribute ``hex_l`` removed in favor of ``hex`` which is now long (6
  hexadigit long). Use ``hexs`` for the possibly shortened version to 3
  hexadigit when possible.
- ``LONG_HEX_COLOR`` and ``SHORT_HEX_COLOR`` are not available in
  module's scope anymore, but are moved in each corresponding format's
  attribute (``Hex.regex``, and ``HexS.regex``).
- ``RGB``, ``HEX``, ``HSL`` helper classes output now again basic python
  types output (tuples and strings). And thus are unaware of any formats.
- in a similar way, ``xxx2yyy`` conversion functions are unaware of any
  format and work with basic python types and returns basic python types.

Author: vaab

README.rst

@@ -131,7 +131,11 @@ Reading values
 Several representations are accessible::
 
     >>> c.hex
+    '#0000ff'
+    >>> c.hexs
     '#00f'
+    >>> c.web
+    'blue'
     >>> c.hsl  # doctest: +ELLIPSIS
     Hsl(hue=0.66..., saturation=1.0, luminance=0.5)
     >>> c.rgb
@@ -158,12 +162,12 @@ Or the hue, saturation and luminance of the HSL representation::
     >>> c.luminance
     0.5
 
-A note on the ``.hex`` property, it'll return the smallest valid value
-when possible. If you are only interested by the long value, use
-``.hex_l``::
+A note on the ``.hex`` property: it'll return the 6 hexadigit, if you
+needed the version of this format that allow short 3 hexadigit when possible,
+use ``hexs`` format::
 
-    >>> c.hex_l
-    '#0000ff'
+    >>> c.hexs
+    '#00f'
 
 
 Modifying color objects