Make duplicate class property names a syntax error

Per the spec in section 13.5 under the 'Static Semantics: Early Errors' section:

"...It is a Syntax Error if PrototypePropertyNameList of ClassElementList
contains any duplicate entries, unless the following condition is true for each
duplicate entry: The duplicated entry occurs exactly twice in the list and one
occurrence was obtained from a get accessor MethodDefinition and the other
occurrence was obtained from a set accessor MethodDefinition..."

This makes esprima throw an error when it encounters such a syntax error.

Perf and coverage runners are happy with the change

https://code.google.com/p/esprima/issues/detail?id=421