Build wheels for macOS

Also optimize builds and clean up

Make Context context manager exit to previously current context

Additionally:

• New class BaseDecoder (buggy, see #40)
• New module functions: current_context, sample_size, sample_length
• Update docs for more readability
• Setter functions for class write-only properties are now written with @setter decorator

Implement MessageHandler

From this release, the source files are moved to src.

Additional changes include:

• Complete methods involving ChannelConfig and SampleType
• Add AuxiliaryEffectSlot to module's __all__ for documentation exposure (and from palace import * but who uses that anyway)

Implement AuxiliaryEffectSlot

Also add some methods to Context and Source, and test Source read-write properties.

Add legal informations of bundled dependencies

0.0.7

Add legal information of bundled dependencies

Build against OpenAL with stereo angles fixed

See kcat/openal-soft#390.

Test Travis deployment

0.0.5

Pump version to 0.0.5 to test deployment