Big cleanup for 4.1.0: initial something
[artifact] Remove namespace from __hash__()

This artifact doesn't know anything about namespaces.
[pomreader] Implement find_parts method
[pomreader] Use find() in find_parts()
[dependency] Initial impl
Move most of the code for creating artifacts from coordinates to AbstractArtifact
[artifact] Remove Ivy code

Ivy specific code will be in separate module
[exclusion] Initial impl
[artifact] Move get_xml_str() and get_xml_element() to AbstractArtifact
[artifact] get_parts_from_mvn_str() is static method now
Code deduplication in Dependency and Exclusion
[dependency] Add support for exclusions
[dependency] Fix get_xml_str()
[plugin] Initial implementation
[dependency] Show exclusions element only if there are some
[extension] Initial implementation
[pom] Cleanup
[pom] Add support for dependencies
[pom] Add support for plugins
[pom] Add support for extensions