Readonly field formatter

Add the readonly keyword to fields that are provably never written
to. Only mark private and internal fields, and the following
are considered "writing" to a field:

- Assignments (_field = 5)
- Compound assignments (_field += 5)
- ref/out calls (Method(ref _field))
- ref of parent to extern method. While not technically required
  the fields can possibly change (even when marked readonly)
- internal fields that are exposed via InternalsVisibleTo an
  assembly that is not in the input project
- fields marked with [Import] or [ImportMany]
- fields of struct types that have methods called on them

Fix #104

Author: ChadNedzlek

src/Microsoft.DotNet.CodeFormatting.Tests/Microsoft.DotNet.CodeFormatting.Tests.csproj

@@ -68,6 +68,7 @@
       <SpecificVersion>False</SpecificVersion>
       <HintPath>..\packages\System.Collections.Immutable.1.1.33-beta\lib\portable-net45+win8+wp8+wpa81\System.Collections.Immutable.dll</HintPath>
     </Reference>
+    <Reference Include="System.ComponentModel.Composition" />
     <Reference Include="System.Composition.AttributedModel, Version=1.0.27.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL">
       <SpecificVersion>False</SpecificVersion>
       <HintPath>..\packages\Microsoft.Composition.1.0.27\lib\portable-net45+win8+wp8+wpa81\System.Composition.AttributedModel.dll</HintPath>
@@ -113,6 +114,7 @@
     <Compile Include="Rules\NewLineAboveRuleTests.cs" />
     <Compile Include="Rules\PrivateFieldNamingRuleTests.cs" />
     <Compile Include="Rules\NonAsciiCharactersAreEscapedInLiteralsRuleTests.cs" />
+    <Compile Include="Rules\MarkReadonlyFieldTests.cs" />
     <Compile Include="Rules\UsingLocationRuleTests.cs" />
   </ItemGroup>
   <ItemGroup>