fixed github issue # 240. This creates the key in the format of HHmm and they are rewritten after 24 hrs. This is to avoid the filling of the redis. Now the format for the value of cpu/ram/hdd is (%usage, date_of_month). The front end has to now check for this data as well
renamed minute to timeKey
added null check for context
creating different values for different key combinations
Merge branch 'github-240' into github-260

Conflicts:
	contrib/src/main/java/com/datatorrent/contrib/machinedata/DimensionGenerator.java
fixed issue #240 and #260. Added documentation for machine data application