Verdi command line for comments

[aiida-bot]

Originally reported by: Giovanni Pizzi (Bitbucket: pizzi, GitHub: giovannipizzi)

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Add verdi commands to comment nodes and to visualize comments of other users.

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• Bitbucket: https://bitbucket.org/aiida_team/aiida_core/issue/87

[aiida-bot]

Original comment by Andrea Cepellotti (Bitbucket: acepellotti, GitHub: cepellotti):

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Implemented in e417b46

[aiida-bot]

Original comment by Giovanni Pizzi (Bitbucket: pizzi, GitHub: giovannipizzi):

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Merge after small conflict.
Added the verdi comment. It is now possible to show, add, update and remove the comments of a node.
The remove of a comment is restricted to the user proprietary of the comment itself (although on the verdi shell he could do it).
Fixes issue #87.