Add spelling correction methods

Includes BaseBlob.correct() and Word.spellcheck()

Also, Word.pluralize() and Word.singularize() return Word objects
Add parsing functionality
Add BaseTokenizer.itokenize method

Returns a generator that generates tokens on-demand
DRY up Blobber initialization
Add Naive Bayes classification.

Adds a text.classifiers module with NaiveBayesClassifier and a basic feature extractor.
Lots of docs updates.
Use language detection in translation

If no from_lang is specified, use language detection.
Also, update NLTK.
Deprecate clean_html
Bump version 0.5.3 -> 0.6.0

Also, some minor docs fixes.