Version 0.4.0

Thanks to @xavArtley for several contributions, and to @lebedov for bugfixes.

New features:

• Now Python2 compatible (#225)
• Audio player widget (#215,#221)
• FileInput widget (#207)
• General support for linking Panel objects, even in static exports (#199)
• New user-guide notebooks: Introduction (#178), Links (#195).

Enhancements:

• Improved Pipeline (#220, #222)

Bug fixes:

• Windows-specific issues (#204, #209, etc.)
• Various bugfixes (#188, #189, #190, #203)