Ignore the maven target directory.

All maven build artifacts are put into the target directory,
and what "mvn clean" does is delete the target directory and
all subdirectories.
Ignore eclipse project files and directories.

The eclipse project files and directories are generated from the pom.xml
by m2e, and when the pom is changed m2e will updates these files to
reflect the pom.
Merge branch 'steinarb-git-ignore-maven-and-eclipse-artifacts'