Added pop() method to ConfigTree and added KeyError to ConfigMissingException

[jjtk88]

Hi there,

I've added a pop() method to the ConfigTree class, and I'm 99% sure that I've done it correctly. Wanted to submit a pull request so that I don't have to keep patching locally. :-) I tried to cover all the edge cases in the test case, but I may have miss something.

Additionally, I added KeyError to the inheritance list for ConfigMissingException so that you can use it more like a dictionary, like:

c = ConfigTree()
..
try:
    v = c['my.key']
except KeyError:
    do_something()

The only thing I'm not entirely sure of is line 225-227. It works, although I wasn't sure if you guys had a more preferred way to split/join key names

My commit msg:

Added pop() method to ConfigTree that works like dict.pop(). Added test
cases as well

Also, appended KeyError to ConfigMissingException so that exceptions can
be caught upstream via normal dict() syntax

[coveralls]

Coverage increased (+0.03%) to 97.956% when pulling f359910 on jjtk88:master into 30c9a51 on chimpler:master.

[darthbear]

Looks good! Thanks @jjtk88 for the PR!

[darthbear]

Pushed to pyhocon==0.3.36. Thanks again!