Add new wrsMemoryGroup with objects:
-- wrsMemoryTotal
-- wrsMemoryUsed
-- wrsMemoryUsedPerc (% of used memory)
-- wrsMemoryFree

Additionally:
-- update of MIB
-- update Makefile
-- update init.c

NOTE: I tried to implement these objects with sysinfo function. However,
sysinfo doesn't provide information about amount cached memory, which is
included in used memory.

Signed-off-by: Adam Wujek <adam.wujek@cern.ch>