Fixes for dropped parentheses around function calls and comma expressions

Example code where the issue is visible:
(function(){}()); --> the outer set of parens are dropped
(a=b,c=d,e=f); --> the outer set of parens are dropped

Both the issues are visible when node.to_ecma() is used.
Unittests for the dropping parens bug (Issue #42)
Merge pull request #1 from miketaylr/fix-dropped-parents-unittests

Unittests for the dropping parens bug (Issue #42)
Merge pull request #44 from harig/fix-dropped-parens

Fix for issue #42 - missing parentheses in some expressions