Disallow wildcard imports (W0401).

* Also flag unused names from wildcard imports (W0614).
Refactor wildcard imports.

* Drop wildcard; state imported names explicitly.
* Import each module only once.
* Drop unused imports.
* Fix broken imports: Explicitly import modules
  previously implicitly imported by wildcard.
Merge pull request #673 from Renelvon/no_wildcard_imports

Fix imports (part 2). Disallow wildcard imports (W0401, W0614)
Upgrade to pybitcointools 1.1.15.
Merge pull request #691 from Renelvon/bump_pybitcointools

Upgrade to pybitcointools 1.1.15
Factor GUIDMixin definition in a new file.

* Abstract initialization of GUIDMixin inside
  initialization of CryptoPeerConnection
* Coerce string to GUIDMixin when adding contact.
Support unicode GUIDs.
Test GUIDMixin.

* Also test CryptoPeerConnection subclassing.
Update documentation.