Disable spark module by default

Hi,

The mvn build was not working out of the box (which should be the case, in my opinion) so I fixed the maven profiles and documented them.

I'm really new to the project, might be that I did something wrong with the spark module (then, consider this as an issue)

I got this error message:
```
[ERROR] Failed to execute goal on project optiq-spark: Could not resolve dependencies for project net.hydromatic:optiq-spark:jar:0.4.17-SNAPSHOT: Failure to find org.apache.spark:spark-core:jar:0.8.0-20130911.235955-2 in http://repo.pentaho.org/artifactory/repo was cached in the local repository, resolution will not be reattempted until the update interval of pentaho has elapsed or updates are forced -> [Help 1]
```