Add field aliases

[andrykonchin]

Closes #466

Changes:

• added alias option for the field method

Example

class User
  # ...
  field :createdAt, :datetime, alias: :created_at
end

user = User.new

user.created_at # => ...
user.created_at = Time.now
user.created_at? # => true
user.created_at_before_type_cast # => ...

Reason

It allows to generate setter/getter/<name>? and <name>_before_type_cast instance methods for specified attribute alias.

Notes

These aliases aren't supported in where conditions (e.g. you cannot use 'created_at.gt': Time.now - 1.hour - use createdAt.gt instead). Dirty API also doesn't support aliases.