bump version to 0.2.8

1. replace generateSignedUserTransactionHex with generateRawUserTrans…

…action + signRawUserTransaction (#17)

2. add CHANGELOG.md

add generateSignedUserTransactionHex in untils.tx (#14)

* bump version from 0.1.7 to 0.2.0

* 1. only export func: generateSignedUserTransactionHex
2. fix bug: new Uint8Array([...hashSeedBytes, ...rawUserTransactionBytes]) will get [[0],[0]] when sdk is called from test_dapp in chrome

* fix bug: the value of non-exists key in javascript is undefined, not null

add generateSignedUserTransactionHex in untils.tx (#14)

* bump version from 0.1.7 to 0.2.0

* 1. only export func: generateSignedUserTransactionHex
2. fix bug: new Uint8Array([...hashSeedBytes, ...rawUserTransactionBytes]) will get [[0],[0]] when sdk is called from test_dapp in chrome

* fix bug: the value of non-exists key in javascript is undefined, not null

doc: add readme