Attention mechanism + tests

Also refactoring: Fork now inherits from Parallel
Bidirectional network is functional
PEP8
Merge branch 'master' into attention

Conflicts:
	blocks/recurrent.py
	blocks/sequence_generators.py
Remove unnnecessary `push_initialization_config`
Refactor, return glimpses, write docs
Documentation fixes
Fix docs
BaseRecurrent becomes a brick
Add mask to the attention mechanism
Merge branch 'master' into attention

Conflicts:
	blocks/initialization.py
Various changes and bug fixes in the attention mechanism

Including:
- changed order of dimensions for weights
- pass **kwargs to the super's constructor
- get_dim and initial_glimpses methods
Hanling of mask in recurrent bricks
Add `get_dims`: syntax sugar for `get_dim`
Debug printing of shapes
A bit more docs and new Mixer brick
First working AttentionTransition
Fix docs and simplify `dict_subset`
PEP8
Change attention transition interface