Applies $scriptpath variable loading in psake.ps1.

[fabriciomurta]

This same approach is already used in the SaltarelleCompiler's 'psake.ps1' script.

[erik-kallen]

Sorry, but this pull request is to big, and it contains changes with which I disagree.

1. packages/ content. I prefer to have these files committed; I realize there is a tradeoff, but I like being able to clone a repo and then have everything I need to build it. Especially in this case, where there are dependencies that are not on nuget.org (the compiler alpha packages).
2. .gitattributes (though it doesn't really matter) IMO this should be set by default in all git installations.
3. Hardcoded version. If you want an alpha build, create a tag called 3.0-alpha.