refactors user's registration

Simplifies process and fields: 

* In: username
* Out: first_name, last_name, phone, nickname, use_nickname?

Closes #213

spec/models/user_spec.rb

@@ -30,6 +30,12 @@ describe User do
     expect(subject).to be_valid
   end
 
+  describe "#name" do
+    it "is the username when the user is not an organization" do
+      expect(subject.name).to eq(subject.username)
+    end
+  end
+
   describe 'preferences' do
     describe 'email_on_debate_comment' do
       it 'should be false by default' do
@@ -44,45 +50,6 @@ describe User do
     end
   end
 
-  describe 'use_nickname' do
-    describe 'when true' do
-      before { subject.use_nickname = true }
-
-      it "activates the validation of nickname" do
-        subject.nickname = nil
-        expect(subject).to_not be_valid
-
-        subject.nickname = "dredd"
-        expect(subject).to be_valid
-      end
-
-      it "calculates the name using the nickname" do
-        subject.nickname = "dredd"
-        expect(subject.name).to eq("dredd")
-      end
-    end
-
-    describe 'when false' do
-      before { subject.use_nickname = false }
-
-      it "activates the validation of first_name and last_name" do
-        subject.first_name = nil
-        subject.last_name = nil
-        expect(subject).to_not be_valid
-
-        subject.first_name = "Joseph"
-        subject.last_name = "Dredd"
-        expect(subject).to be_valid
-      end
-
-      it "calculates the name using first_name and last_name" do
-        subject.first_name = "Joseph"
-        subject.last_name = "Dredd"
-        expect(subject.name).to eq("Joseph Dredd")
-      end
-    end
-  end
-
   describe "administrator?" do
     it "is false when the user is not an admin" do
       expect(subject.administrator?).to be false
@@ -152,9 +119,8 @@ describe User do
       expect(subject.organization.name).to eq('org')
     end
 
-    it "deactivates the validation of first_name and last_name, and activates the validation of organization" do
-      subject.first_name = nil
-      subject.last_name = nil
+    it "deactivates the validation of username, and activates the validation of organization" do
+      subject.username = nil
       expect(subject).to be_valid
 
       subject.organization.name= nil