This is to ease use of mahout with eclipse

- add .cache to .gitignore
- change buildtools pom to override java 1.4, to prevent eclipse warning
- change math-scala pom to include scala source folders, to prevent
build error in mahout-h2o & allow correct building by scala-ide plugin

There is still one vexing error in using eclipse, in mahoout-mrlegacy, a
source folder points outside the project (root mahout/src/conf).  Mvn
creates a source entry but eclipse doesn't recognize it.