Section updates

[mpsonntag]

This PR updates Section methods and adds many tests.

• reorder is moved from base to Section since it makes no sense to have it in Document or Property. Closes Refactor reorder() method #267.
• Adds an extend method for child Sections and Properties. Closes change value related behaviors #237.
• Appending multiple Sections or Properties has been removed from the append method to mirror Property append functionality and since extend now serves this need.
• Changes __init__ set attributes order: In the previous set attribute order, the repository attribute was overwritten with None by the super.__init__ after it had been set.
• Adds strict flag to merge method. Closes Add section.merge() strict keyword #270.
• Fixes set parent = None bugs in remove and insert.
• Adds a name check for existing child on insert.
• Adds various section and section save/load tests.

Don't be afraid, its mainly tests!

[coveralls]

Coverage increased (+1.2%) to 73.625% when pulling 31b8ff1 on mpsonntag:secUpdate into 9d8af75 on G-Node:master.

[JuliaSprenger]

I tested the append and extend functionality, which works fine for me. For the merge on section level, this does not consider additional attributes like the definition of a section. Do you think it would make sense in the strict=True case to check for matching definitions of sections?