Add PICKITEM for ByteArray into VM

[ixje]

What current issue(s) does this address, or what feature is it adding?
neo-project/neo-vm#98

How did you solve this problem?
write code

How did you make sure your solution works?
run VM test vectors

Are there any special changes in the code that we should be aware of?
no
Please check the following, if applicable:

• Did you add any tests?
• Did you run make lint?
• Did you run make test?
• Are you making a PR to a feature branch or development rather than master?
• Did you add an entry to CHANGELOG.rst? (if not, please do)

[ixje]

StackItem.New did not create a ByteArray when receiving a str input value therefore failing some local tests. The VM test-vectors did not cover this.
C# ref: https://github.com/neo-project/neo-vm/blob/bfb68c806f52ed773ae7badff3b652e96e257c61/src/neo-vm/StackItem.cs#L95-L98

[coveralls]

Coverage decreased (-0.01%) to 86.764% when pulling 217c6e5 on ixje:pickitem-vm into bcb46c4 on CityOfZion:development.