README.md directions when Chocolatey install fails with 407

Chocolatey installation can fail behind a proxy requiring authentication due to [HTTP 407](http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.8). This change provides an additional command line invocation for those cases that uses the default credentials for proxy authentication prior to downloading.
Merge pull request #842 from atifaziz/patch-1

README.md directions when Chocolatey install fails with 407
pass invalid namespaces up to executor as it keeps it's own list
catch ex CS0246 and extract namespace
added a const for CS0246
added extra null check and Any() check
added tests
changed InvalidNamespaces property to be immutable and backed by a field
Merge pull request #850 from filipw/bugfix/849

Prevent incorrect namespace from breaking your REPL
Added failing test case
Fixed bug
red: added RoslynScriptEngineTests.ShouldAllowUseOfScriptHostExtensionMethods to demonstrate that extension methods on script host do not work