Establish reserved transformation context attribute names

[fabiocarvalho777]

Establish reserved transformation context attribute names. The idea is to promote gathering transformation metadata that is very common to any extension and have an easy and standardized way to refer to them. A good example of it is #220. These names must respect #168.

They must be defined in an interface, called ReservedAttributes, to be implemented by TransformationTemplate. Also, the TransformationContext implementation must not allow the extension developers to misuse them, meaning, setting them with values of wrong type. For example, application name must be a String. Probably an enumeration should be kept in ReservedAttributes mapping attribute name and type.

These reserved attributes can be set using TransformationTemplate.set() (see #223), or the regular TransformationUtility.setContextAttributeName(String).