Js gettext fix

Hallo :)
I made my first tests with this angular-gettext in last Friday and everything seemed to work as intended. I continued today and started using 'gettext()' method inside a controller, because I needed to translate jqPlot axis labels for jqPlot config object. My project uses jqplot + anguar ui-chart which is a small directive build a top of the jqPlot. These similar situations occur often if your project contains some other jquery plugin wrappers for Angular and you need to provide configs through JavaScript.

The problem was obvious when the 'gettext()' method returns always the same string and it was only used to extract translations.  I first thought that I was missing something and that 'angular-gettext' would have some dependency to some 'gettext' library, but it didn't.

So I started to make some re-factoring to the code and changed the 'constant("gettext")' to 'factory("gettext") which returns translated string if available. I made some testing and noticed that it didn't have any side effects to text extraction at least in my tests.

This update would only make one change, it requires that you need to DI 'gettext' into your controller, but in Angular environment I see this a small thing, because gettext() -method it self works as it should. This also gives a workaround to 'ng-if' issue which I noticed. Now you can add such translations through controller variable.

I also moved 'trim' method from directive inside a filter, because I saw that a better place for that. 

I made tests for my changes and also for the 'trim' filter change.

There was one thing that would require some thought from you, it is 'gettext.coffee' test descriptions would not match now for those that are currently written.

I am not sure that would this update also give a way to fix this kind of issues? 
https://github.com/rubenv/angular-gettext/issues/67

What do you think about these changes?

Br, 
Markku Roponen