Fix for empty array checking

https://github.com/typicaljoe/taffydb/pull/22
sum() breaks when columns are missing from some objects

https://github.com/typicaljoe/taffydb/issues/23
Caching bug fix

https://github.com/typicaljoe/taffydb/pull/28
has function's return should default to false

Defaulting to true resulted in returning false positives.
Create README
Updates to tests

https://github.com/typicaljoe/taffydb/pull/30
crash caused by custom filter fixed

https://github.com/typicaljoe/taffydb/issues/32
Merge pull request #35 from DanielRuf/master

fixed some issues and merged most of the pull requets