Add support to sign tx with single key or array of keys

[robertdavid010]

Pop the first element out if its an array, and fallback to the first key if the array runs out of keys.
Closes #237

[ttmc]

Hi @robertdavid010

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[future-is-present]

Travis tests are failing.
What about if you change just line 239 for:

const privateKey = privateKeys[index] ? privateKeys[index] : privateKeys[0] 

[future-is-present]

Hi @robertdavid010 any update here?

[robertdavid010]

Have to discuss with current project to confirm signing. Can request addressing of this maybe today or tomorrow.

…

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