Fixing single character tags

[mcrot]

I've just opened issue #81, because I cannot use single character tags with the current version
of tagulous. Here is a pull request which might fix the issue.

First I've added some tests to show the problems I have. Second, I've replaced the function
utils.split_tree_name(name) with simpler code.

Do I miss something or can it just be implemented by using the split() method of strings, as I did?
One drawback of my solution is that you cannot use a NULL character in tags anymore,
but that shouldn't be problem - although there also would be a solution for that.

What do you think? The tests are passing, also the new ones I've added.

[coveralls]

Coverage decreased (-0.2%) to 95.271% when pulling b132e9a on mcrot:develop into 62d6528 on radiac:develop.

[coveralls]

Coverage decreased (-0.2%) to 95.247% when pulling b132e9a on mcrot:develop into 62d6528 on radiac:develop.

[mcrot]

Hm - how can I improve the coverage? It seems like that most uncovered code is related to
old Django versions.

[mcrot]

@radiac , I would like to ask you whether you could review these changes and help me to increase the test coverage a bit.

I think this is a useful change, but I don't know how to increase the test coverage. It seems like all lines not covered are not covered because they are only needed when using old Django versions or South.