Add support for DateTime attributes

Expects DateTime objects to be encoded with ISO-8601 standard
see http://www.w3.org/TR/NOTE-datetime for more inforamtion.

Also converts from Date and Time objects to DateTime.
Add support for Float attributes

Uses .to_f method on object, can convert from all objects supporting this method
Merge pull request #4 from nicolas-fricke/add_float_support

Add float support
Merge pull request #3 from nicolas-fricke/add_date_time_support

Add support for DateTime attributes
Increase messaging sleeps.
Disable unreliable messaging tests.
Add list attribute type.
Add default values for response attributes.

Some attribute will be initialized by stubs. Use defaults to avoid
method missing errors called on these attributes.
Also create request from operation for stubbed responses.
Expand erroneous response error message.
Include data hash in no stub found error message.
Add stub specs for update operations.
Fix problem with update operation stubs.

Also introduce support for blocks as `with` parameters for stubs to allow dynamic decisions.
Add option to test of an operation was called and how many times.
Add AmbiguousStubsError and specs for it.
Update README and CHANGELOG in order to release 0.19
Update shield path.
Bump version 0.20.0
Use more requires instead of autoload.
Get attributes and attribute_types from superclass to inherit attributes in subclasses.