Added free and total memory to /system/stats.json.
Added free and total memory to /system/stats.json.

This patch expose the number bytes of total and free memory in
the /system/stats.json.

Review: https://reviews.apache.org/r/17569

dforsyth/mesos,chhsia0/mesos,Gilbert88/mesos,yuquanshan/customizedMesos,craimbert/mesos,Aman-Jain-14/customizedMesos,andschwa/mesos,gsantovena/mesos,jpeach/mesos,dforsyth/mesos,yuquanshan/customizedMesos,zmalik/mesos,yuquanshan/customizedMesos,abudnik/mesos,Gilbert88/mesos,zmalik/mesos,asamerh4/mesos,craimbert/mesos,dforsyth/mesos,asamerh4/mesos,reneploetz/mesos,Aman-Jain-14/customizedMesos,Aman-Jain-14/customizedMesos,gsantovena/mesos,dforsyth/mesos,chhsia0/mesos,gsantovena/mesos,abudnik/mesos,zmalik/mesos,kaysoky/mesos,neilconway/mesos,gsantovena/mesos,chhsia0/mesos,chhsia0/mesos,zmalik/mesos,zmalik/mesos,asamerh4/mesos,Gilbert88/mesos,kaysoky/mesos,abudnik/mesos,chhsia0/mesos,andschwa/mesos,verizonlabs/mesos,abudnik/mesos,neilconway/mesos,dforsyth/mesos,reneploetz/mesos,neilconway/mesos,andschwa/mesos,abudnik/mesos,reneploetz/mesos,neilconway/mesos,asamerh4/mesos,andschwa/mesos,Aman-Jain-14/customizedMesos,chhsia0/mesos,andschwa/mesos,gsantovena/mesos,andschwa/mesos,jpeach/mesos,jpeach/mesos,Gilbert88/mesos,kaysoky/mesos,asamerh4/mesos,reneploetz/mesos,chhsia0/mesos,yuquanshan/customizedMesos,verizonlabs/mesos,verizonlabs/mesos,zmalik/mesos,andschwa/mesos,yuquanshan/customizedMesos,reneploetz/mesos,kaysoky/mesos,craimbert/mesos,jpeach/mesos,neilconway/mesos,yuquanshan/customizedMesos,neilconway/mesos,jpeach/mesos,neilconway/mesos,craimbert/mesos,reneploetz/mesos,yuquanshan/customizedMesos,jpeach/mesos,kaysoky/mesos,craimbert/mesos,Aman-Jain-14/customizedMesos,abudnik/mesos,Aman-Jain-14/customizedMesos,Gilbert88/mesos,zmalik/mesos,craimbert/mesos,verizonlabs/mesos,dforsyth/mesos,Gilbert88/mesos,jpeach/mesos,kaysoky/mesos,asamerh4/mesos,verizonlabs/mesos,dforsyth/mesos,verizonlabs/mesos,kaysoky/mesos,abudnik/mesos,asamerh4/mesos,verizonlabs/mesos,gsantovena/mesos,gsantovena/mesos,Gilbert88/mesos,reneploetz/mesos,craimbert/mesos,Aman-Jain-14/customizedMesos