Add INDEX path and (type, path) to nodes table

Specification in `20130605091736_create_nodes` to index nodes.path was not
valid. Also by adding (type,path) index, worker process and accordion tree get
fast! Furthermore I set key length limit for indexing because an index key can
be up to 767 bytes in InnoDB.
Merge pull request #46 from yohoushi/index

Index was missing