Allow EditableFileUploadAttribute to be required

Added a custom validator so that EditableFileUploadAttribute can be
marked as required without throwing an exception.
Fixed a FormatException

Fixed a FormatException caused by an incorrect format string index.
Strongly Typed DisplayExtensions

Add stronger typed versions of DisplayContent helpers. Avoids an "Unable
to cast object of type 'System.Linq.Expressions.UnaryExpression' to type
'System.Linq.Expressions.MemberExpression'" exception in certain
circumstances.
Merge pull request #263 from nimore/EditableFileUpload

Allow EditableFileUploadAttribute to be marked as Required
Merge pull request #264 from nimore/DisplayExtensions

Strongly Typed DisplayExtensions
Correcly append edit=drag on links with hash (fixes #265)
Refactored nuget config to use xdt transform method, now NoZip removes any zip vpp config which should make it possible for zip and nozip to coexist (fixes #259)
Assembly version 2.5.3
Updated n2cms.config version to 1.1 and tweaked config source
Update readme.txt
Update readme.txt
Rename readme.txt to readme.md
Update readme.md
working on code examples
working on updating content of readme
Finished final draft of n2cms readme

I love this content management system.  I want to see it gain more followers and contributors.  I think this will help a ton.  Keep up the great work guys.
added api section and move user experience to the top
found a type in readme
updated webform page example
making examples as accurate as I can