Prevent duplicate external projects in solution

Fixes #52.
Enable "PlatformSpecificOutputFolder" by default

Fixes #56.
Create LICENSE
Update test path for new Protobuild
Add Windows Phone 8.1 built-in definitions
recompress and bootstrap Protobuild with Windows Phone 8.1 definitions
Recompress and rebuild
Implement support for RemoveDefine in service dependencies
Allow Protobuild services to be recommended instead of required

This allows services to be recommended instead of required, which means
they can be disabled with --disable, or by enabling a conflicting
service.

The primary use for this is recommending sane defaults for various
platforms, while still allowing users to override the defaults.
Allow services to be used in external projects
Ensure external projects filtered by services are included in solutions

This also removes the "[not(@Guid=preceding::Project/@Guid)]" filter in
the solution XSLT with proper validation, as this was causing issues
when a previous entry was not active, followed by one that was (the
latter would not be written out because of the preceding clause).
Allow only Any CPU on Windows Phone 8.1 Platform
Rebuild Protobuild
Fix issue where solution generation would ignore platform
Update protobuild to set ToolsVersion add Microsoft.Common.Props to wp8.1 and updates the minimum vs version
update Product Version and Visual Studio version for wp81
Add Microsoft.Windows.UI.Xaml.CSharp.Targets to wp8.1
Merge pull request #61 from aljim/master

Add Visual Studio 2013 properties for wp8.1
Rebuild and recompress Protobuild
Update README to link to new site