Add support for classifier

• Complexity:

  Intermediate

When building C and C++ projects there are multiple dimensions to consider, for example 32 vs 64 bits and GCC version.

For RedHat 4.x and 5.x use different GCC versions, but GCC should be compatible within each range.

While artifactId can be used to model one of these dimensions, it is not convenient to use artifactId for both.

The classifier concept seems like a good addition to support his use case.