Improvement `remote.request_book_offers` parameters passing

As the example  in doc reference:
var request = remote.request_book_offers({
gets: {
'currency':'XRP'
},
pays: {
'currency':'USD',
'issuer': 'rvYAfWj5gh67oV6fW32ZzP3Aw4Eubs59B'
}
});
It will raise `TypeError: Cannot read property 'currency' of
undefined`. Fixed issue with parsing parameters
Merge pull request #34 from chgwei/develop

Improvement `remote.request_book_offers` parameters passing