Handle servers without defined checks gracefully

If a haproxy config doesn't specify any health checks for a server, the
exported CSV value will contain "no check" instead.
Consolidate label naming

The term "instance" both conflicts with the reserved label prometheus
will apply and also doesn't match the HAProxy terminology.
Merge pull request #4 from prometheus/handle-no-check

Handle servers without defined checks gracefully
Merge pull request #5 from prometheus/rename-instance-to-server

Consolidate label naming
Export error and HTTP metrics

This change adds support of the following additional metrics:

* econ: haproxy_connection_errors
* eresp: haproxy_response_errors
* wretr: haproxy_retry_warnings
* wredis: haproxy_redispatch_warnings
* hrsp_{2xx,3xx,4xx,5xx}: haproxy_http_responses