Fix path used when storing segments to HDFS

Hi,

as discussed in https://groups.google.com/d/topic/druid-development/IwYHYdNx_no/discussion, I fixed the HDFS path for segments. 

I use another date format for the intervals which does not contain ':' symbols. I also replace ':' characters with '_' in the version string. This should workaround the already known issue with colons in HDFS (https://issues.apache.org/jira/browse/HDFS-13)

I tested this with hadoop-2.0.0-cdh4. The segments are now pushed to HDFS properly and loaded to compute nodes properly.

Thanks for accepting the modification.
-- Jan