Automate builds and NuGet publications

[julienmoumne]

When #8 and #9 are completed, it will be possible to publish builds to NuGet.

The goal of this ticket is to automate the process of building and publishing a new version of the project to NuGet.

Piwik relies on travis-ci for such tasks. For simplicity, It would be nice to use the same tool.

Some preliminary tests on travis-ci has been performed by @tetsuo13, see results in #8

The implemented solution should be easily extended to include #7

[julienmoumne]

See https://github.com/piwik/piwik-dotnet-tracker/issues/65 for a first step