str/repr, join, and setting - for arrays

[benkehoe]

I batched these together but can split them out/discard as desired.

__str__ and __repr__ methods

Pretty straightforward, some hoops to jump through for Python 2 support, as __str__ should return a str, not unicode (#48 is relevant).

Join

Add a join() method for combining two pointers. Works with strings and parts as well (can be more restrictive on types if you want). Also supports the / magic method (__truediv__ in Python 3, __div__ in Python 2), like pathlib.

Setting - for arrays

A pointer like /foo/- represents the element past the end of an array. Calling set() with one of these pointers should append to the array.

[coveralls]

Coverage remained the same at 100.0% when pulling d811454 on benkehoe:master into 896102d on stefankoegl:master.

[coveralls]

Coverage remained the same at 100.0% when pulling d811454 on benkehoe:master into 896102d on stefankoegl:master.

[coveralls]

Coverage remained the same at 100.0% when pulling d811454 on benkehoe:master into 896102d on stefankoegl:master.

[stefankoegl]

Great, thanks a lot!