Add custom loaders for function notation

[adriangonz]

Fixes #17

Changelog

• Move most of Model to BaseModel
• Remove cls pointer and simplify how custom functions are bound to the class
• Add loadmethod decorator when using function notation
• Add context field to share object instances between different methods (useful when using the function notation)
• Improve how pipelines / models get initialised in MLServer
• Add example for custom model with custom artifacts

TODO

• Call __init__ in MLServer for decorated classes (to be done on issue Save class definition (instead of instance) #91)
• Clear model's context field before saving artifact