Implement support for ranges

[pspanja]

Ranges can be implemented as an extension with a custom Term token, but it would still be nice to have them in the base syntax. The way Elasticsearch defines them looks pretty nice:

• inclusive: date:[2012-01-01 TO 2012-12-31]
• exclusive: tag:{alpha TO omega}
• one side unbounded: age:>10 age:>=10 age:<10 age:<=10

[thePanz]

A first implementation for value:[123 TO 555] is in #14