Necessary interfaces to implement text parsers.

## Parser

The package `org.apache.lucene.queryparser.flexible.parser` contains interfaces that should be implemented by the parsers. Parsers produce QueryNode Trees from a string object. These package still needs some work to add support to for multiple parsers. Features that should be supported for the future, related with the parser: - QueryNode tree should be able convertible to any parser syntax. - The query syntax should support calling other parsers. - QueryNode tree created by multiple parsers.