License clarification

I added the license headers to the files that were missing them under the assumption that they should be LGPL 2.1 or greater.  Along that same line several of the files include the GPL v2 or greater header instead and I'm wondering if you intended that the gem have a dual license or if that was just due to starting out GPL and switching to LGPL later on.

Obviously the copyright is yours so I just thought I'd draw your attention to it in case you wanted to address it.

```
$ licensecheck -r * | grep -v LGPL
api_tests/tc_test_user.rb: GPL (v2 or later)
rakefile.rb: GPL (v2 or later)
ts_all_tests.rb: GPL (v2 or later)
ts_local_vars.rb: GPL (v2 or later)
zbxapi/defines.rb: GPL (v2 or later)
zbxapi/result.rb: GPL (v2 or later)
zbxapi/revision.rb: *No copyright* GENERATED FILE
```