Snmp checks

Agent check to allow SNMP polling
Add pysnmp and pysnmp-mibs dependency
Based on the list of metrics specified in the yaml file, will poll the different snmp enabled device.
Support SNMP v1,v2 and v3
SNMP Counter metrics are transformed as a rate and gauge are reported as gauge.

Tests are not passing due to pylint errors fixed in another PR #923 

Could you organize your imports the same way it's done here ?
https://github.com/DataDog/dd-agent/blob/omnibus_bundling/checks.d/ntp.py#L1-L8