Document complex validations better

[pascal-za]

Some validations in the migrant DSL can get very complex and generate interoperability issues with different Ruby versions.

See presidentbeef/brakeman#25

Possible solutions

• Expanded documentation to give further validation examples (serving as 'templates' if you like)
• Simplified multiple validations (perhaps writing the field declaration as a block or something)

structure do
  some_really_validated_field do
    validates :presence
    validates :inclusion, :in => %w(Apple Pear)
  end 
end

Just throwing some ideas around right now.