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
Remove unused goog.require
Merge pull request #2092 from fredj/unused-require

Remove unused goog.require
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
Test that a custom geometry works
Merge pull request #2095 from ahocevar/test-geometry-name

Test that a custom geometry works