Optional usage of environment variable and increased timeout

Hello Hiroyoshi & Johannes,

I've added the option to use a environment variable to specify the stanford core directory. This is usefull, when you don't want to specify the location of the directory in the code. Instead it's just a configuration.

Additionally I've increased the timeout for loading the stanford core nlp to 60 seconds, because on slower PCs the default timeout of 30 seconds is to slow, especially if you use bigger classifier training files, than the default ones.

Cheers, Felix