Add EntityValidationException

OPEN - task 68: Create Custom Exception for entity validation 
http://github.com/DevOpsDistilled/OpERP/issues/issue/68
Extend EntityValidationException

OPEN - task 68: Create Custom Exception for entity validation 
http://github.com/DevOpsDistilled/OpERP/issues/issue/68
Make validate() throw EntityValidationException

OPEN - task 68: Create Custom Exception for entity validation 
http://github.com/DevOpsDistilled/OpERP/issues/issue/68
Make validate() throw EntityValidationException

OPEN - task 68: Create Custom Exception for entity validation 
http://github.com/DevOpsDistilled/OpERP/issues/issue/68
Catch EntityValidationException instead of Exception

OPEN - task 68: Create Custom Exception for entity validation 
http://github.com/DevOpsDistilled/OpERP/issues/issue/68
Declare Custom Exception for Entity Validation

Merge branch 'iss68-custom-exception' into develop