Removed usage of precomputed data

In order to be able to tweak the word vectors based on the surrounding
sentence, it is not possible to make these precalculations. It slows
down the running by a factor 400.
Added replacement of word with sentence mean

Replaces the embedding lookup of a word with the mean of all words in
the current sentence.
Logic fix and speed improvement

Now omit the current word when calculating the mean value.
Moved some code out of a loop to increase speed performance.
Added mean as a config

Can now set the usage of mean feature from command line by passing
-feature "mean".