[FEATURE] Allow to specify type and override even simple types

This adds the type attribute to the Mapping annotation to allow
overriding of the detected type.

The EntityMappingBuilder is changed so it will use a Mapping annotation
even on properties of a simple type, before such an annotation would
silently be ignored.
Merge pull request #3 from Flowpack/mapping-tweaks

[FEATURE] Allow to specify type and override even simple types