Use native VSSDK ZipPackageCompressionLevel

... instead of repackaging with VsixCompress.

I found digging thorugh *C:\Program Files (x86)\MSBuild\Microsoft\VisualStudio\v12.0\VSSDK\Microsoft.VsSDK.targets* in the `<Target Name="CreateVsixContainer"` it uses the `ZipPackageCompressionLevel` property when creating the VSIX. Whether this is a new addition in the VS2013 SDK I don't know. Anyway, this seems to get defaulted to to `NotCompressed`.

Setting `<ZipPackageCompressionLevel>Normal</ZipPackageCompressionLevel>` in WebEssentials2013.csproj results in the same level of compression as using VsixCompress and results in a quicker build time (as the VSIX doesn't get extracted and re-compressed).