Add Username+Password support to MSBuild Task

[SimonCropp]

similar to this #21 (comment)

[asbjornu]

Isn't this supported through the use of environmental variables?

[GeertvanHorrik]

That will be difficult if you have multiple accounts for multiple repositories. But to be honest, I only use GitVersion on the server, and then I prefer the command line tool anyway.

For this to work via environmental variables, we need something like GitVersion[RepoName]UserName.

[asbjornu]

With TeamCity, environment variables are per build configuration which is scoped to one VCS (repository) anyway, so within that context, there shouldn't be a problem. But in other contexts, I agree that they probably need some kind of namespacing or other scoping mechanisms.