dtype handling and setting id

[mpsonntag]

This PR

• Introduces the new_id() method to Document, Section and Property and adds the corresponding tests. Closes Add new_id method for Document, Section, Property #262.
• Setting a dtype now also supports odML style tuple types and adds tests Closes [dtypes] odML custom tuples are not valid dtypes #254.
• When initializing a Property now only valid dtypes will be set. Adds corresponding tests. Closes [Property] Init allows invalid dtypes #253.

[coveralls]

Coverage increased (+0.3%) to 71.986% when pulling 4d956da on mpsonntag:refid into 4a10c2c on G-Node:master.

[achilleas-k]

Approving (but waiting for regex fix) :)