Added priorities for encoders and normalizers

Bug fix: [no]
Feature addition: [no]
Backwards compatibility break: [no]
Unit test passes: [yes]
Specs passes: [yes]
Replaced unit test classes removed: [yes]
Behat scenario passes: [yes]
Checkstyle issues: [no]*
ChangeLog updated: [yes]
Documentation PR: 
Fixes the following jira:
 - ...

There is no possibility to add encoders in the current pim version, as the pim_serializer.normalizer.get_set_method is appliable to any object will be selected each time.

This PR fixes this by adding a priority to the pim_serializer.encoder and pim_serializer.normalizer tags