Added setter and deleter methods for children property

[gregreich]

Let's do this as a pull request, so you also get the tests.

[coveralls]

Coverage decreased (-0.2%) to 99.613% when pulling 169b404 on gregreich:children_setter into 1c8e087 on c0fec0de:master.

[gregreich]

The build fails for python 2.6, because I use assertRaises as a context manager (which is handy if you test assignments with assertRaises), which has only been added in 2.7:

"Changed in version 2.7: Added the ability to use assertRaises() as a context manager." (from Python unittest doc)

Can you accept this build failure?

[c0fec0de]

Please see https://github.com/c0fec0de/anytree/blob/master/tests/helper.py
I implemented my own assert_raises because of that...

[coveralls]

Coverage decreased (-0.2%) to 99.613% when pulling e661468 on gregreich:children_setter into 1c8e087 on c0fec0de:master.

[coveralls]

Coverage increased (+0.01%) to 99.806% when pulling a652d6e on gregreich:children_setter into 1c8e087 on c0fec0de:master.

[coveralls]

Coverage increased (+0.01%) to 99.806% when pulling d4b54a5 on gregreich:children_setter into 1c8e087 on c0fec0de:master.