Overhaul of advanced features

Origin of:

- composite types
- one of type
- entity definition
- exclusive properties
- examples folder

Syntax update for:

- one of type
- referencing
- templates properties (variable property name)
Add traits to reserved keywords

Update HAL example
Add reserved keywords classification

Examples cleanup
Code review fixes & clarifications
one of type as data structure
Markdown header entity definition clarified

Including some minor typo fixes
Include instead of inherit for mixins

To prevent any confusion between mixins and inheritance, mixins are now
using keyword `include` in stead of `inherit`
Fix minor typo
Merge pull request #2 from apiaryio/zdne/advanced-features

Overhaul of advanced features