Create NullFieldException to throw when required fields are Null.

OPEN - task 45: Creation of Item with null value for Product and Brand
possible 
http://github.com/DevOpsDistilled/OpERP/issues/issue/45
Check if fields are Null, if so, throws NullFieldException

OPEN - task 45: Creation of Item with null value for Product and Brand
possible 
http://github.com/DevOpsDistilled/OpERP/issues/issue/45
Add throws declaration for NullFieldException

OPEN - task 45: Creation of Item with null value for Product and Brand
possible 
http://github.com/DevOpsDistilled/OpERP/issues/issue/45
Define implementation for NullFieldException if thrown

OPEN - task 45: Creation of Item with null value for Product and Brand
possible 
http://github.com/DevOpsDistilled/OpERP/issues/issue/45
Parse Item price from input textfield

OPEN - task 45: Creation of Item with null value for Product and Brand
possible 
http://github.com/DevOpsDistilled/OpERP/issues/issue/45
Allow only numeric value in "Price" field and invalidate Item creation
with Null fields.

OPEN - task 45: Creation of Item with null value for Product and Brand
possible 
http://github.com/DevOpsDistilled/OpERP/issues/issue/45
Merge branch 'iss45-prohibit-null-item-creation' into develop