Added Sign extension for assembly certificate signing 

This adds support for PFX certificate signing
(a.k.a Authenticode not to be confused with strong naming)
using the standard Windows SDK SignTool
Example usage:
```
Task("Sign")
    .IsDependentOn("Clean")
    .IsDependentOn("Restore")
    .IsDependentOn("Build")
    .Does(() =>
{
    var files = GetFiles(solutionDir + "/**/bin/" + configuration + "/**/*.exe");
    Sign(files, new SignToolSignSettings {
            TimeStampUri = new Uri("http://timestamp.digicert.com"),
            CertPath = "digicert.pfx",
            Password = "TopSecret"
    });
});
```
Well signing is only one of the things SignTool can do
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Valid commands:
        sign       --  Sign files using an embedded signature.
        timestamp  --  Timestamp previously-signed files.
        verify     --  Verify embedded or catalog signatures.
        catdb      --  Modify a catalog database.
        remove     --  Reduce the size of an embedded signed file.
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