Allow to specify an objectToExportTo for goog.exportSymbol

This allows users to build ol3 without anything exposed in the
global namespace. This can e.g. be useful for creating an ol3
AMD module, by simply using a build configuration with
"define('ol',function(){var o={};%output%return o.ol;});" as
"output_wrapper".
Document the new namespace option
Pass around single config object
Additional doc for the namespace option

People should be discouraged from exporting to single letter namespaces to avoid collissions with the symbols generated by the compiler.
Merge pull request #2082 from ahocevar/custom-exports-target

Allow to specify an objectToExportTo for goog.exportSymbol