Add support for /v1/agent/check/... and /v1/agent/checks.

I have tested by:
- creating both scripted and TTL checks
- failed/passed scripted check
- pass_/warn/fail TTL check
- show all checks:

{u'c-1': {u'Node': u'shahhaqu-w530', u'CheckID': u'c-1', u'Name': u'TTL test', u'ServiceName': u'', u'Notes': u'', u'Status': u'critical', u'ServiceID': u'', u'Output': u'hixthere3'}, u'c-0': {u'Node': u'shahhaqu-w530', u'CheckID': u'c-0', u'Name': u'SSH port ping', u'ServiceName': u'', u'Notes': u'', u'Status': u'passing', u'ServiceID': u'', u'Output': u''}}