Sample plugin for commit message validation.

Shows how to add additional commit validation to
Gerrit using a plugin.

Validation can be based on:
- user performing the Git command
- Gerrit project
- any of the commit elements
- ref-name

Change-Id: Ie283e29feee319fd4f3b7e3f3b833dbe1dbf5c9e
Signed-off-by: Luca Milanesio <luca.milanesio@gmail.com>
Signed-off-by: David Pursehouse <david.pursehouse@sonymobile.com>