Added NuGet Add/Remove/Has source methods
1. AddSource
    - Adds NuGet package source using the specified name, source &amp; settings to global user config
2. RemoveSource
    - Removes NuGet package source using the specified name &amp; source from global user config
3. HasSource
    - Checks if NuGet package source exists in global user config using the specified source &amp; settings
Initial NuGet Sources Add test classes
Added RedirectStandardOutput, HasSource now uses ProcessRunner, added tests.
ProcessSettings now has RedirectStandardOutput(false as default) used by ProcessRunner,
when set to true the process output can be retrieved with IProcess/ProcessWrapper GetStandardOutput().
NuGetSource is now testable by using Cake standard ProcessRunner instead of System.Diagnostics.Process
Removed duplicated "cake" code, added settings and post action to Tool.Run
HasSource now uses the standard Tool<T>.Run
Added extra Tool<T>.Run overload which allows to override ProcessSettings
and specifying a Action<IProcess> postAction which if specified is triggered
after process exit.
Added more AddSource tests
Added logging to GetStandardOutput
Merge pull request #125 from devlead/addsource

Safer handling of private NuGet feeds under CI.