Added #create and #create! methods to AWS::Record::AbstractBase

Added two methods to `AWS::Record::AbstractBase`

Assuming a class like this:
````ruby
    class Book << AWS::Record::Model
      string_attr :title
      validates_presence_of :title
    end
````

#create will create a new object and save it.
````ruby
    book = Book.create(:title => "The big book of tests")
    book.persisted? # => true
````

#create will return an invalid object if the attributes are not valid
````ruby
    book = Book.create()
    book.persisted # => false
    book.valid? # => false
````

#create! will try to create a new object and raise an exception if the object is invalid.
````ruby
    book = Book.create!()  # => InvalidRecordError
````