Closes #25 Adds error if invalid user name or password given at /login, adds specs for error, POST Sessions#create now redirects to login_url if invalid credentials given.
Resolves #28 link_url_format_valid? will only validate when :link is true.
Resolves #26 /signup shows errors when User is invalid.
Resolves #29 and #30 Adds error messages to POST #create and PATCH #update.