GH-511: change DocumentLSTMEmbeddings to use LSTM instead of GRU

[kubapok]

add other available RNN types to choose from

[alanakbik]

Hey this is great, thanks for adding this.

However, I am getting an error now ('DocumentLSTMEmbeddings' object has no attribute 'embedding_length') if I call this code:

from flair.models import TextClassifier
classifier = TextClassifier.load('en-sentiment')

There may a problem with backwards compatibility with serialized models.

[alanakbik]

@kubapok thanks for commiting this - I'll merge this into the release branch and take alook at the error with the serialized models!