Made JavaScriptContext.Collect() public

...because VS2008 running regression tests for .Net 2.0 build cannot
access it otherwise.  I don't understand why VS 2012 can get access to
an internal method in another DLL.
Added nunit.runners nuget reference

...so they will be installed automatically and so references to them can
be made from the readme instructions.
Updated projects for VS2008 and x64.  README

Updated README.txt for current build methods.
Updated Visual Studio 2008 projects to work with Gyp-built v8.
Updated Visual Studio 2012 project to work with x64 builds.
Merge pull request #9 from oliverbock/master

Made JavaScriptContext.Collect() public