Do not call lager as it is not guaranteed to be present

I am not sure what approach is the best for a debug log entry, so only
commenting the lines at the moment.

Fixes the following warning:
```
Unknown functions:
  ...
  lager:debug/2
```
Fix `elarm:get_alarm_by_id`...

and .. fix type spec of `elarm:get_alarms`.

This was not caught by Dialyzer.

The `{ok, ...}` bit in the return type of `get_alarms` was already
used in the tests. See
https://github.com/esl/elarm/blob/3350c195c23e371dbe5be6e7ed858e9bf016df63/test/elarm_subscr.erl#L65
Fix unused variable warning

Addresses https://github.com/esl/elarm/pull/33#discussion_r16947807
Merge pull request #33 from lucafavatella/fix-dialyzer-warnings

Misc fixes