proposal to differentiate between setting property key or property key path 

As commented on this commit, https://github.com/JaviSoto/JSRestNetworkKit/commit/8a183e4402e99d2c3e02a0927abfe61e981fe1c3#commitcomment-1409577

I think is interesting to differentiate between using valueForKeyPath and valueForKey to improve performance (overall because JSBaseEntity should be extremely used in any app).

So, this is my proposal to let the user indicate if is going to parse using valueForKey or valueForKeyPath.

Feel free to discard the pull if you think is not necessary ;)