safe eval

[mblondel]

In Parallel, joblib uses eval for its pre_dispatch, which can be unsafe in some circumstances. Possible solutions:

• For Python >= 2.6, the use of the ast.literal_eval would help http://docs.python.org/library/ast.html#ast.literal_eval
• For older Python versions, specifying explicite globals and locals in the eval would help a bit, especialy if we provided a builtins with a reduced set of builtins.
• Use a callable (function of n_jobs)

[fcharras]

Was fixed in #1327