Added versioning track header implementation

At this point of the implementation the version number must be updated manually in pom.xml and ctct_api.properties.

I tried to use properties-maven-plugin, but a warning is raised by maven "'version' contains an expression but should be a constant." and the token ${sdk.version} doesn't get replaced with the value in ctct_api.properties, when running mvn install. See below

<groupId>com.constantcontact</groupId>
<artifactId>constantcontact</artifactId>
<version>${sdk.version}</version>

with ctct_api.properties file:
sdk.version=2.1.5

I will continue to investigate the issue, but for now i thinks it's ok to merge.