Create CONTRIBUTING.md

initial contributing document
Update CONTRIBUTING.md
Update CONTRIBUTING.md

[ci-skip]
downgrade maven requirement to 3.0

close #12
[maven-release-plugin] prepare release sass-maven-plugin-2.0-RC2
[maven-release-plugin] prepare for next development iteration
add junit 4 dependency

add some tests

  - a Compass + webapp integration test
  - a unit test stub for Resource

run verify instead of test for coveralls

setup up for integration testing and unit testing using the same samples.

add Sass only integration test

also do comparison tests ignoring line-ending of compile and expected
CSS and minor refactoring & docs

add an integration test that uses the <resource> element

docs + todo's

drop guava 18, it's causing a conflict

exception:

```
java.lang.IllegalAccessError: tried to access method
com.google.common.collect.MapMaker.makeComputingMap(Lcom/google/common/base/Function;)Ljava/util/concurrent/ConcurrentMap;
	 from class com.google.inject.internal.Annotations$AnnotationChecker
```

guava is still available as transitive through the maven-core

update tests

Update .travis.yml

the default travis install fails as there is no deployed snapshot; compile will get most dependencies as well.

Update .travis.yml

initially skip all tests

Update .travis.yml

add a misconfigured test

update integration test

MavenSassIntegrationTest and MavenCompassIntegrationTest fail being
unable to find the produced files (.css and .css.map)

seems related to setting  List<Resource> resources to an
Collections.emptyList(); in d093a80e16aa488daedf354a1a26f6b85f4a4d6b

better unit testing + resource cleanup

enable clean as part of the integration tests

and change line endings in expected result
and build + compiler config

Update .travis.yml

run tests with -e insted of -X and for coverage run `verify` only

javadoc fixes

don't build test javadoc for now