Added description of default_serializer_options 

In example it's used to set up root: false for controller
Add support for specifying the serializer for an association as a String.

This enables the deferral of the resolution of the serializer class to prevent NameError exceptions due to reference cycles between serializer classes.
Update README.md

Spelling and murkup mistakes fixed.
Fix global ``` self.root = false ```

Using the on_load hook does not change the defined root on
Serializer#inherited.

Related to #179
Add failing test to show behavior of custom serializer with nils.
reduce 2x map; fixes datamapper eager loading
Merge pull request #210 from tpitale/master

Reduce double map to once; Fixes datamapper eager loading.
test cases by fblee

Merge remote-tracking branch 'fblee/master'
serialization test: respect root settings when root object is nil
Don't fail if object is nil, render null
Add ActiveSupport::DescendantsTracker to Serializer and ArraySerializer
Remove caching of Serializer constants to allow ActiveSupport::Dependency reloading to work
Merge pull request #215 from mulder/feature/reloading

Serializer Reloading via ActiveSupport::DescendantsTracker
changes to be able to specify multiple attributes with keys
Update README.md file to better describe scope.

This commit also corrects the code about deactivating globally the root name of arrays.
Merge pull request #218 from phcoliveira/patch-1

Update README.md file to better describe scope.
Merge pull request #209 from ismaelga/attributes_with_keys

Able to specify keys on the attributes method
Merge pull request #172 from vladson/patch-1

Added description of default_serializer_options to README.md. Fixes #168
Merge pull request #167 from GateGuru/feature/specify-association-serializers-as-strings

Add support for specifying the serializer for an association as a String...
No longer recommend installation from git