BCL proxies added.
Included Proxies.Bcl to release build.
BCL proxies cleaned up and update for last JSIL changes.

--HG--
extra : source : a606c3916eae40d19a64ef596d090dbffd1bc91e
Added SimpleTestCasesSourceForTranslatedBcl (empty yet). It runs test with translated mscorlib and System.Core using Proxies.Bcl and throwOnUnimplementedExternals=false.

--HG--
extra : source : 94809eace594d8e859cc6a9c50cb0b570554f283
extra : amend_source : c45207921eb28d5b2632ed041bc16cf2b2aa20a8
Fix foldertestsources crashing if the source folder is missing.