Deduplicated NuGet GetDefaultToolPath
Added a core NuGetToolResolver for the any code that utilizes the nuget.exe like for example the NuGet Tool<T>:s
Extended it so it
  1. Caches path if found
  2. Not only checks via globber pattern but now does several steps
    1. NUGET_EXE environment variable
    2. Globber "./tools/**/NuGet.exe"
    3. If exists in environment path
Merge pull request #141 from devlead/nugetfromenv

Deduplicated NuGet GetDefaultToolPath
Added #addin directive for NuGet addins
Makes it possible to load and reference addins via NuGet, i.e #addin "Cake.AliaSql"