Extract the class PomTidy that tidies a POM.

Separate the code for tidying a POM from the Mojo stuff. This is a
preparatory step for adding a verfiy goal.
Transfer POM as String instead of StringBuilder.

String is a value object and therefore better suited for transferring
data, because it cannot be altered.
Extract IndentCalculator.

Avoid duplicate code.
Use unit tests instead of integration tests.

This provides a faster feedback cycle and the JUnit messages for failed
assertions are more meaningful.