Add numbackends to postgres dbstat metrics

As discussed in mperham/inspeqtor#26.  This makes the number of
connections, or `numbackends` available when writing rules. For example,
to check that your connection pool hasn't exploded unexpectedly:

```
if postgresql:numbackends > 50 for 4 cycles then alert
```
Treat numbackends as a guage, not a counter
Merge pull request #35 from sorentwo/postgres-numbackends

Add numbackends to postgres dbstat metrics
LookupService should always return something non-nil, fixes #36
Merge branch 'master' of github.com:mperham/inspeqtor
Fix tests
Better init.d logging, fixes #32
postgresql changes
initd changes
bump version
Update LICENSE
Merge pull request #37 from seuros/patch-1

Update LICENSE
new import path
Merge branch 'master' of github.com:mperham/inspeqtor
Add Travis-CI build, fixes #16
Install some deps
Need to prepare
changes
Use postgres as the default account, for Travis CI
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