[pom_editor.py] initial version of pom_editor rewrite
[test] Removed invalid test for multiple parents

Removed test for adding parent to POM which already had a parent,
because it expected pom_editor to do it, but POM with multiple parents
is not valid. Will add negative test for it later.
[test] delete blank line in test expectations
[pom_editor.py] refactoring, retain original formatting

Refactored pom_editor.py to be more object-oriented and to retain
indentation and formatting of original POM.
[macros] modify macros to use pom_editor.py
[macros] typo in doc comment
[pom_editor.py] allow removing multiple deps/plugins/modules at once
[test] test for error messages not just error codes

Testing just return codes doesn't fail for script's syntax errors or
uncaught exceptions.
[test] tests for nonexistent poms
[pom_editor.py] fail correctly on invalid XML
[pom_editor.py] fix pom_add_parent
[doc] small note about pom macros and namespaces
[pom_editor.py] refactoring
[pom_editor.py] allow matching multiple elements in pom_xpath_*
[test] more tests for pom_editor
install new pom_editor
[pom_editor.py] extra xml in pom_add_plugin
[test] test for extra xml content in pom macros
[pom_editor.py] print usage on error
[macros] minor fixes in macro comments