Add validation rules for nuspec metadata

[imlucas]

Forked from #6.

We need to provide clean error messages before we get to spawning binaries. For example, in #6 we need to cleanly indicate the problem is spaces in the package id cannot include whitespace. From the nuspec docs the package id is:

The unique identifier for the package. This is the package name that is shown when packages are listed using the Package Manager Console. These are also used when installing a package using the Install-Package command within the Package Manager Console. Package IDs may not contain any spaces or characters that are invalid in an URL. In general, they follow the same rules as .NET namespaces do. So Foo.Bar is a valid ID, Foo! and Foo Bar are not.

As well, we should make the app model should make the default conform to the spec.