Move settings fixture into test_assets.py

This fixture is only used in one place, so let's just use it in that one
place.
DictMock is only used in test_api.py
Move tests alongside tested modules

This commit moves our unit tests into 'test' packagaes alongside the
modules they are intended to test. This has a number of advantages:

- predictable test locations: the tests for module foo.bar are always at
  foo.test.bar_test
- 1:1 mapping module to test file: this helps enforce the testing of
  isolated units
- easy overview of module coverage: we can quickly and easily identify
  modules that are not tested
Merge pull request #1682 from hypothesis/unit-tests-with-modules

Move tests alongside tested modules
gitignore: Don't ignore all dotfiles
Use prospector for static analysis rather than flake8

Prospector is a cover-all static analysis tool that runs the same tools
as flake8, plus a number of others including pylint.
Ignore a couple of false-positive pylint warnings
Remove unreachable code

This code was "removed" shortly before launch when the realtime features
were disabled. This commit removes this code entirely rather than
leaving dead and unreachable code in the repository.

This commit will probably be reverted in due course.