Add format argument to fromXDR()

[bartekn]

Additional format argument specifying input format.

// decodes the buffer directly
xdr.TransactionResultPair.fromXDR(aBuffer)

// decode the buffer after first decoding from hex
xdr.TransactionResultPair.fromXDR(aBuffer, 'hex')

// decode the buffer after first decoding from base64
xdr.TransactionResultPair.fromXDR(aBuffer, 'base64')

Close #2 To support https://github.com/stellar/js-stellar-base/issues/32

[nullstyle]

:+1: