A lot of changes

Hi,

So i found your i18n library yesterday and i've decided that i'm going to use it
for my new project. But it had a few bugs, and it felt like some pieces was still missing.

What i did was try to improve the build process, how the i18n package is installed into
nuget projects and how gettext strings are extracted.

Major changes:
1) Replace i18n.PostBuild.exe with a msbuild task. Also fixed a couple of bugs. Unit tests are included. A major benefit is that you get detailed error messages if something goes wrong and it's easy to extend later on with maybe optional warnings such as missing translations.
2) Upgrade gettext to latest version.
3) Improve build script.
4) Add i18n.targets that is automatically into projects by nuget. This ensures that
gettext strings are extracted at compile time.
5) Fix compiler warnings and cleanup code formatting (just minor indenting and xml markup fixes).
6) Upgrade nuget dependencies to their latest respective versions.

I'm sure there is more, you'll just have to look at the PR.

You are probably going to recieve more commits tomorrow. My hope is that we can close 
of some outstanding bugs and adding more unit tests so that we are sure that
everything works as expected.

Anyways, keep up the good work :)