Adapt Stanbol DataFileProvider to propertly work with enabled Security

Using the MainDataFile provider with enabled SecurityManager cuases exceptions because the anonymouse user does not have read access to the stanbol datafiles directory.

java.security.AccessControlException: access denied (java.io.FilePermission

read)
at java.security.AccessControlContext.checkPermission(AccessControlContext.java:374)
at java.security.AccessController.checkPermission(AccessController.java:549)
at java.lang.SecurityManager.checkPermission(SecurityManager.java:532)
at java.lang.SecurityManager.checkRead(SecurityManager.java:871)
at java.io.File.exists(File.java:731)
at org.apache.stanbol.commons.stanboltools.datafileprovider.impl.MainDataFileProvider.getDataFile(MainDataFileProvider.java:274)
at org.apache.stanbol.commons.stanboltools.datafileprovider.impl.MainDataFileProvider.getInputStream(MainDataFileProvider.java:156)
at org.apache.stanbol.commons.opennlp.OpenNLP.lookupModelStream(OpenNLP.java:541)
at org.apache.stanbol.commons.opennlp.OpenNLP.initModel(OpenNLP.java:478)
[..]