Upgrades (work in progress)

fixes #100
adds AdaptiveLearningRate Observer
refactored ErrorMinima from EarlyStopper (so it can be used without early-stopping)
depends on https://github.com/clementfarabet/lua---nnx/pull/22

Still need to:
 * [x] integrate AdaptiveLearningRate to recurrentlanguagemodel.lua
 * [x] write RecurrentDictionary doc
 * [x] write ErrorMinima doc
 * [x] write AdaptiveLearningRate doc
 * [x] complete Dictionary doc
 * [x] write SentenceSampler doc
 * [x] write RecurrentVisitorChain doc
 * [x] finish benchmarking the new changes to the RecurrentDictionary/SentenceSampler
 * [x] move zeroGradParameter calling into Visitors (fixes RNN output layer bug)
 * [x] write changes to Visitor doc (zeroGradParameters)
 * [x] unit test AdaptiveLearningRate