Add raw tx building examples

[ixje]

What current issue(s) does this address, or what feature is it adding?
A common question in the #python support channel is how to build raw transactions alá neon-js that can be send over RPC using the sendrawtransaction method.

How did you solve this problem?
Add a commented file to \examples\ with 2 examples 1) building a ContractTransaction 2) building an InvocationTransaction

How did you make sure your solution works?
n/a

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]

@localhuman is setting the Gas attribute for InvocationTransactions mandatory?

[coveralls]

Coverage increased (+0.002%) to 82.048% when pulling cabfb60 on ixje:raw_tx_examples into 5a324b3 on CityOfZion:development.

[localhuman]

I don't think so, unless it will cost more than the 10 free gas.

One comment, you can import address_to_scripthash from neocore.bin.cli I think rather than adding it in the example.

[localhuman]

Very helpful!

[ixje]

That's what I thought (regarding the GAS question). As for the helper method, I was already surprised I couldn't find a utility function, didn't think of looking inside the neocore cli. I'll now stick with my current version as it also does the UInt160 conversion I need for the later functions, but thanks for pointing it out 👍