add decimal value vector support (Decimal9, Decimal18, Decimal28Dense,
Decimal38Dense)

- initally implemented by James Duong
- add macros to make byteswap portable on Linux and Mac
- add getvalueAt to get string representaion of decimal

TODO: Decimal28Sparse and Decimal38Sparse does not work at this point
Merge pull request #4 from xiaom/decimal-valuevector

add decimal value vector support