dealing with >=, <= and date stuff in NoZebra

dealing with >=, <= and date stuff in NoZebra

- if the user search on >= or <=, fix a bug removing the < and >
- if the user search on a numeric value (mainly for dates search), retrieve only numeric results

Example : previously Date >=2005 returned "printed in 1976", as "printed" was > than 2005
now it don't.
note that Date >=1900 returns "printed in 1976" as NZ search is a always wrdl search (and, as you know 1976 > 1900 ;-) )

Signed-off-by: Chris Cormack <crc@liblime.com>
Signed-off-by: Joshua Ferraro <jmf@liblime.com>