Add QuickFixResponse base class
Make FindUsagesResponse inherit QuickFixResponse
Re-add GotoImplementationResponse

This will play nicer with testing. I had problems with testing json
with DeserializeJson<IEnumerable<QuickFix>> so I decided to try with
this route instead.
Add GotoImplementationTests

Running this does not work for me on mono. It fails with the following
exception:

mono --debug /usr/lib/nunit/nunit-console.exe OmniSharp.Tests.dll -run OmniSharp.Tests.GotoImplementation

NUnit version 2.5.10.0
Copyright (C) 2002-2010 Charlie Poole.
Copyright (C) 2002-2004 James W. Newkirk, Michael C. Two, Alexei A. Vorontsov.
Copyright (C) 2000-2002 Philip Craig.
All Rights Reserved.

Runtime Environment -
   OS Version: Unix 3.2.0.40
  CLR Version: 4.0.30319.1 ( 2.10.8.1 (Debian 2.10.8.1-1ubuntu2.2) )

ProcessModel: Default    DomainUsage: Single
Execution Runtime: Default
Selected test(s): OmniSharp.Tests.GotoImplementation
.Loading test.cs
F
Tests run: 1, Errors: 1, Failures: 0, Inconclusive: 0, Time: 0.798994 seconds
  Not run: 0, Invalid: 0, Ignored: 0, Skipped: 0

Errors and Failures:
1) Test Error : OmniSharp.Tests.GotoImplementation.GotoImplementationTests.Should_find_usages_in_same_file
   System.NullReferenceException : Object reference not set to an instance of an object
  at OmniSharp.Parser.BufferParser.ParsedContent (System.String editorText, System.String filename) [0x0000d] in /home/mika/koodi/OmniSharpServer/OmniSharp/Parser/BufferParser.cs:19
  at OmniSharp.GotoImplementation.GotoImplementationHandler.FindDerivedMembersAsQuickFixes (OmniSharp.GotoImplementation.GotoImplementationRequest request) [0x00000] in /home/mika/koodi/OmniSharpServer/OmniSharp/GotoImplementation/GotoImplementationHandler.cs:28
  at OmniSharp.Tests.GotoImplementation.GotoImplementationTests.Should_find_usages_in_same_file () [0x0007b] in /home/mika/koodi/OmniSharpServer/OmniSharp.Tests/GoToImplementation/GoToImplementation.cs:41
  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
  at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00000] in <filename unknown>:0