provide property filtering on .pom files placed in local repo

When maven installs an artifact, it's pom is also copied into the artifact's directory. Unfortunately, if the pom contains a property reference (e.g. $

), this will not be replaced upon copying the pom file.
I've created a patch for the install plugin that switches on property filtering by setting a mojo parameter "filteringEnabled". Since this defaults to "false", backward compatibility is kept 100%.

Some implementation notes:

• the dirty work is done in FilteredProjectArtifactMetadata.java, the property resolution code has been inspired by ResourcesMojo.
• I've added a unit test, that replaces $ {basedir}

  with the value of a system property.

• since "svn diff" does not handle binary files, src/test/resources/unit/basic-install-test-with-filtering/target/maven-install-test-1.0-SNAPSHOT.jar is not included in the patch. This file is the same as
  src/test/resources/unit/basic-install-test/target/maven-install-test-1.0-SNAPSHOT.jar

Since my knowledge of Maven API is more than limited, there might be a more elegant way to provide this feature ... but it works! I'd be happy to see this in a future release of maven.