Use d3 to display mem utilization.

Get mem utilization data, and modify it for stacked layout
Draw stacked graph
Adjust width and margin
change number of data points to 300
Update graph at intervals and render path transition
Change update frequency, window length, and no of ticks
Add text labels for values
Display legend for mem util, display swap usage
Merge branch 'issue#371' of https://github.com/sujeetsr/rockstor-core into issue#520_snapshot_delete_bug
improve umount message