Update model for mem info and generate migration file.
Implement API support for memory utilization.

Retreive raw data from /proc/meminfo and save it to the db appropriately.
Add API url, /sm/sprobes/meminfo/ and make the information available via
ListAPIView.
Closes #115 backend.

Enhance pagination using ListAPIView and get parameters from global settings.
Implement a custom renderer for downloading raw probe data. The renderer is
generic enough but pilot it with memory utilization data.
Memory utilization

Display used, free and total memory
Memory utilization

Add download button with default times for 24hrs
Display total, used and free memory and swap
Memory utilization

Modify colors for used and free
Merge pull request #149 from sujeetsr/issue#112_memory_utilization

Issue#112 memory utilization