Release 0.7.6 r12writer

[Manfred Moitzi]

• NEW: r12writer.py - a fast and simple DXF R12 file/stream writer. Supports only LINE, CIRCLE, ARC, TEXT, POINT, SOLID, 3DFACE and POLYLINE. The module can be used without ezdxf.
• NEW: Get/Set extended data on DXF entity level, add and retrieve your own data to DXF entities
• NEW: Get/Set app data on DXF entity level (not important for high level users)
• NEW: Get/Set/Append/Remove reactors on DXF entity level (not important for high level users)
• CHANGE: using reactors in PdfDefinition for well defined UNDERLAY entities
• CHANGE: using reactors and IMAGEDEF_REACTOR for well defined IMAGE entities
• BUGFIX: default name=None in add_image_def()