Extracted 'inner' closure as a pickleable class, ParallelExecutor. This addresses the initial concern that the inner closure is not pickleable in #489, but issues still remain that the tasks themselves appear not to be pickleable.

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extra : source : 0e7a8732c0109eb54fe64c6d729e5024acb69673
extra : histedit_source : 32b1057a9621aa66dc7d9674781c69702f060145
Add __main__ so that fabric can be invoked with 'python -m fabric'
Update changelog
Add note about `ecdsa` lib for Paramiko 1.12
Use Python 3 compatible syntax

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extra : amend_source : 6d1af1e0d902810c81b969d57e8949be5a0a0aeb
Supply isMappingType (removed in Python 3); Use reduce from functools
Copy nested implementation as it appears not to have a Python 3 version
Merge parallel-executor
Merge with module-invocable
Merge with python3