Upgrade to pybitcointools 1.1.15

### Motivation:
The latest version of `pybitcointools` packs [improved speed](https://github.com/vbuterin/pybitcointools/commit/be6db939ff033b72099c66c41022cffb91abdc51) and a [revamped testsuite](https://github.com/vbuterin/pybitcointools/pull/38). I just finished a diff between version `1.1` and `1.1.15` without detecting anything that could cause trouble save for one thing: the latest version redefines functions `sum` and `slice`, which shadow python builtins and make `pylint` unhappy. However, there is no usage of these functions in our code; the `pylint` warning is a result of the wildcard imports in `node/ws.py` and `node/orders.py`. Thus, this PR depends on #673.

### Changelog:
* Upgrade to `pybitcointools` version `1.1.15`.
First of all, this line isn't ever executed. There are no `import`s of `ebay-button`. Second, this line *can't* be correctly executed. There is no `ebaysdk` requirement in our `requirements.txt`. So it's basically dead code, and it begs to be removed together with the entire folder. However, I'd like to proceed step by step.

You saw this change in another PR (in #673) because this is where it belongs. That PR is "a dependency" for this one; upgrading directly to `pybitcointools 1.1.15` [breaks the build](https://travis-ci.org/Renelvon/OpenBazaar/builds/36462945). To make the dependency explicit, I branched off of `no_wildcard_imports` which is the branch of #673. The diff against the originating branch is [just one commit](https://github.com/Renelvon/OpenBazaar/compare/Renelvon:no_wildcard_imports...Renelvon:bump_pybitcointools), the last one. However, because GitHub makes a diff against `develop`, thus incuding the changes (and the commits) of #673.

The reason for submitting dependent PRs is that I can pipeline my work. A PR that is marked as dependent on another PR shouldn't be reviewed before the dependencies are merged. When they do, the common commits and changes will go away automatically, and the PR will be fit for review.

I understand this all sounds complicated. @dionyziz is used to that kid of PRs and this is why he is assigned.