Ignore eclipse project files.  Reorganize and document intent.

After reorganizing the .gitignore file I do not have eclipse or intellij project files cluttering up status.
I hope it helps others.

I went from:
```git
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#	liquibase-core/.classpath
#	liquibase-core/.project
#	liquibase-core/.settings/
#	liquibase-integration-tests/.classpath
#	liquibase-integration-tests/.project
#	liquibase-integration-tests/.settings/
#	liquibase-maven-plugin/.classpath
#	liquibase-maven-plugin/.project
#	liquibase-maven-plugin/.settings/
#	liquibase-osgi/.classpath
#	liquibase-osgi/.project
#	liquibase-osgi/.settings/
#	liquibase.iws
#	samples/
```

to pretty clean