"PatriciaTrie" is a "Trie", which is used in "Indexer" to map the feature ID to the index of feature vector.
"Indexer" has a "PatriciaTrie".
Fix: "Size" and "Index" should "*Indexer"'s methods.
Start with simple implementation of trie.
"SimpleTrie" is the internal representation of "Indexer".
Merge branch 'feature/trie' into master
"Trie" should have a getter method and a setter method for node's value.
Separate "Indexer" from "model.go".
Index features with "SimpleTrie".
Fix: "(Trie).Update" returns "true" if the key already exists.
Merge branch 'feature/indexer' into master
TODO: Use the efficient implementation of trie for "Indexer".
TODO: Create interfaces and structs to implement features easily.
TODO: Implement some sparse matrices.
Merge branch 'doc/todo' into master