Replace check_memory script to be more accurate

The current check_memory script only checks free/used RAM.
It does not account for cached and buffered memory.
This would result in, for instance, database servers with a large amount of
cached memory but little free RAM to generate warning/critical alerts while
there would be literally nothing to worry about.

This script takes into account cached and buffered RAM and also provides
better perfdata.
Before:
./check_snmp_mem.pl -H 127.0.0.1 -C shinken -f -w 95,50 -c 99,60

Ram : 95%, Swap : 0% : > 95, 50 ; WARNING
| ram_used=3856652;3856814;4019206;0;4059804 swap_used=4736;1999868;2399842;0;3999736

After:
./check_snmp_mem.pl -w 95 -c 99 -- -v 2c -c shinken 127.0.0.0.1

MEMORY OK: 56.63 % used;
Free => 98212 Kb,
Total => 4059804 Kb,
Cached => 105092 Kb,
Buffered => 1557240 Kb
|ram_free=98212 ram_total=4059804 ram_cached=105092 ram_buffered=1557240
Merge pull request #7 from dmsimard/better_memory

Replace check_memory script to be more accurate