Migrate example for developers

Examples for Developers module provides high-quality, well-documented API examples for a broad range of Drupal core functionality.

I opened a feature request at https://drupal.org/node/2182621 to include also a Migrate example there and posted a first patch with a migrate example module.

Few notes:

• Migrate in core is under heavy development. Most of the work taking place in a sandbox project at https://drupal.org/sandbox/chx/2105305. For this reason this module cannot be tested right now against core but needs to be tested on the code from sandbox.
• The example module assumes that https://drupal.org/node/2182481 will be committed in main migrate module. If not, then the patch needs work to read the destination plugin either from migrate_drupal (if will go in D6) either the plugin will e moved in this module.
• API might still change until reaching a stable stage.
• The 'body' field from the content is not migrated yet due to https://drupal.org/node/2164451. A new patch will be posted as soon as we can migrate fields.