Update CSRF to support Vapor 4 (vapor-community#14)

* Work on converting to use with Vapor 4

* Update to latest vapor rc

* Change package name

* Rename csrf to CSRF

Co-authored-by: Jimmy Arts <Jimmy.Arts@ahold.com>

Provide tag as shortcut for forms in templates (vapor-community#10)

* Provide leaf tag

* Insert information about FormFieldTag into readme

* Make it work with Vapor 3.1.0

* Fit comment in readme to existing style

Merge pull request vapor-community#7 from rb-de0/vapor3

Support Vapor 3.0.0

Merge pull request vapor-community#5 from calebkleveter/master

Added Support for Swift 4

Merge pull request vapor-community#4 from vapor-community/mattmatt/ht…

…tpmethods-access

Makes static properties on HTTPMethod public

Merge pull request vapor-community#3 from vapor-community/mattmatt/cs…

…rf-access-level

Makes CSRF initializer public

Updates to README

Smoothed out some poor grammar and updated some code examples to be accurate.