Bump version to 0.9.0

[berendkleinhaneveld]

This release includes the following features:

• Support for template refs (Add support for template refs #140)
• Hot reloading of components (Hot reloading #146)

And the following fixes and improvements:

• Prevent unnecessary remounts of elements (Prevent unnecessary remounts of fragments in 'if' directives #152)
• Remove the need for a wrapping template element around dynamic slot content (Remove the need for a template element around dynamic slot content #147)
• Keep elements ordered properly (Fix anchor lookup with nested components #155 , Fix anchor lookup with templates #156)
• [PySide] Improve interaction with PySide elements (Block signals when setting an attribute directly #151)
• [PySide] Improve resilience of PySide in light of exceptions (Make PySideRenderer more resilient to errors #154)