Add XDTHelper

Add support for invoking config file changes manually using idiomatic
FAKE syntax.
Add XDTHelper

Add support for invoking config file changes manually using idiomatic
FAKE syntax.
Add XDTHelper

Add support for invoking config file changes manually using idiomatic
FAKE syntax.
Fix proj file, remove autoload.
Update packages
Add XDTHelper

Add support for invoking config file changes manually using idiomatic
FAKE syntax.
More tolerance for missing XDT files, added simple tests.
Fix test reference file.
Add XDTHelper

Add support for invoking config file changes manually using idiomatic
FAKE syntax.
Add XDTHelper

Add support for invoking config file changes manually using idiomatic
FAKE syntax.
Add XDTHelper

Add support for invoking config file changes manually using idiomatic
FAKE syntax.
Merge project fix.
Cross-platform tests. (Mono XML still incompatible with XDT.)

The Mono implementation of System.Xml.XmlAttribute.set_Prefix
is still incompatible with XDT because it's behavior when set to
null does not match .NET.
See https://bugzilla.xamarin.com/show_bug.cgi?id=19426
Merge branch 'XDTHelper' of https://github.com/brianary/FAKE into XDTHelper