Replace sign_transaction utility calls

[youkaicountry]

Replace calls to sign_transaction with a pure python transaction signing solution.

In #4, a python ECDSA library was identified: https://github.com/warner/python-ecdsa

And a pure python serialization library was created in #3.

[youkaicountry]

We use the curve secp256k1, which is present in the library.

[theoreticalbts]

The problems I see are as follows:

• If I do anything other than copy-paste the code from steem-python, I will inevitably get complaints that I'm violating the new requirement that simple_steem_client be a drop-in replacement for steem-python.
• The code in steem-python does careless and inconsistent handling of prefixes. This is one of the reasons it needs replaced, since testnet will have a different prefix and it's not really careful about handling that.
• The code in steem-python has PasswordKey() and BrainKey(). I'm not sure of the compatibility matrix of these features with steemd cli_wallet, tinman get_key_from_seed, and condenser account passwords.
• If this is going to be a drop-in replacement for steem-python, I don't want to drop features that people might be using. But that goes against simple_steem_client being a simplification of steem-python.
• Some of the code I don't understand, such as _derive_y_from_x(). We have at least one signing issue in steemd due to copy-paste "cargo culting" of signing code by someone who doesn't understand it, let's not duplicate that in our Python library.
• Why does base58 need to know about prefixes? I'm not sure.

So what I'm going to do is just copy-paste the bare minimum necessary to get tinman up and running. Basically, PublicKey and PrivateKey. I'm going to get rid of all unnecessary code (including PasswordKey, BrainKey, Address, and uncompressed key handling). I'm going to simplify the interface to be as simple as possible. If we later want to copy-paste larger portions of steem-python for backwards compatibility's sake, let's do it in a different project that uses simple_steem_client as a dependency. For now I'll copy only what we absolutely need.