Add a refactoring to switch between "string" and @"verbatim string"

Also add the necessary infrastructure to unit-test refactorings
Add help links and descriptions to analyzers

For some of them I didn't had any good description that
wasn't a repetition of the text so I avoided to add it
but still added the help link to all of them.

While reviewing all of them I also added the tag to get
greyed-text on analyzer indicating unnecesary constructs:

    customTags: WellKnownDiagnosticTags.Unnecessary

Follow the discussion in #78
Ensure that comments are kept in the refactoring
Merge pull request #94 from vbfox/descriptions-and-help-links

Add help links and descriptions to analyzers
Merge pull request #93 from vbfox/string-types-refactoring

Add a refactoring to switch between "string" and @"verbatim string"