[NuGet] Support version constraints in packages.config file.

NuGet allows you to define a range of package versions that are
allowed in your project.

http://docs.nuget.org/docs/reference/versioning

This is done by editing the packages.config file and adding an
allowedVersions attribute for the installed package. For example,
the following restricts jQuery to be any version from 1.4 up to but
not including version 1.8.

  <package
    id="jQuery"
    version="1.4.1"
    targetFramework="net40"
    allowedVersions="[1.4.1,1.8)" />

The NuGet addin now supports this information when updating the
package. With the above example when updating jQuery the NuGet addin
will now update to jQuery 1.7.2 and not the latest 2.1.1 version.
Previously it would update to the latest version of the NuGet package.
This affects the Update menu items in the Solution window.

Note that if you search for another NuGet package and install a later
version that is outside the allowed range it will still install. This
is the same behaviour as Visual Studio. The allowedVersions attribute
is to prevent you from accidentally updating the version but you can
explicitly pick a later NuGet package and update to it with the Add
Packages dialog.
[NuGet] Use version constraints when checking for package updates.

A version constraint defined in the packages.config file, as shown
below, is now used when checking for package updates from a package
source. Previously always the latest version of the NuGet package
would be displayed as an update in the Solution window.

  <package
    id="jQuery"
    version="1.4.1"
    targetFramework="net40"
    allowedVersions="[1.4.1,1.8)" />

With the above constraint when the 'Check for package updates on
opening a solution' option is enabled in preferences then only jQuery
1.7.2 is shown as available update in the Solutin window.

Previously the latest version of jQuery (currently 2.1.1) would be
shown as an available update. After updating to jQuery 1.7.2 then no
further updates would be shown as available in the Solution window.