[Prepare for v0.1] Enhancement Collection

[JT-Ushio]

Collect a variety of bugs and enhancements.

• add Label_field for classification tasks.
• override __str__ for better print display.
• design universal shadow_padding function for classification, sequence labeling and seq2seq tasks.
• add SeqTokenIndexer, DynamicSeqTokenIndexer.
• add adaptation to dynamic index.
• add adaptation to dynamic get batch.
• add f+b choice to DeepBiLSTMBuilder (default is dy.concatenate([f, b])).
• add .transduce() to DeepBiLSTMBuilder (same as dynet rnn_builder).
• ...
• waiting for the supplement~ : -)