filter out the relation_stats parameter

without this, if you use relation_stats in your configuration, this
happens:

```
CRITICAL   2014-01-16 00:20:27,655 19372  MainProcess     MainThread newrelic_plugin_agent.plugins.postgresql      poll                      L259   : Could not connect to PostgreSQL, skipping stats run: invalid connection option "relation_stats"
```
memcached: fix connection_structures key

In memcached stats output it's called `connection_structures`; can't
find any trace of `connection_struct` in memcached history so this is
probably not a recent renaming, so this change ought not to break
anything.
Clarify log message and demote to INFO

Doesn't really call for action by admin, so WARNING is too much.
add timeout to newrelic api calls

Resolves: #226
update debian init script to be more robost

Add --retry when stopping the agent so that after TIMEOUT (default=5) the process is killed. Previously there was no check at all on wether or not the agent actually exited.

Adds configurability via the standard /etc/default/newrelic_plugin_agent files.
Merge pull request #228 from 3dna/timeout

add timeout to newrelic api calls
Merge pull request #222 from gthb/patch-3

memcached: fix connection_structures key
Merge pull request #234 from trbs/deb_init_timeout

update debian init script to be more robost
Merge pull request #223 from gthb/patch-4

Clarify log message and demote to INFO
Merge pull request #216 from 3dna/fix-postgres-relation-stats

filter out the relation_stats parameter