Do not automatically try to integrate with test libraries.

Since the automatic test library integration functionality requires the test
library to be loaded and this doesn't usually happen until *after* the bundle
is loaded, it makes things simpler if we use `require 'mocha/setup'` to
explicitly setup Mocha when we know the test library has been loaded.

I deprecated the use of `require 'mocha'` in v0.13.0 and I plan to release
this change as part of a major version bump (i.e. v1.0.0), so it ought to
be safe to remove the deprecation in `lib/mocha.rb`.
Provide shortcuts for integrating with specific test library.

Now you can use:

    require 'mocha/test_unit'

OR

    require 'mocha/mini_test'

Instead of:

    require "mocha/integration/test_unit"

    Mocha::Integration::TestUnit.activate

OR

    require "mocha/integration/mini_test"

    Mocha::Integration::MiniTest.activate

Note: both of these assume Mocha itself has been required either explicitly:

    require 'mocha'

OR

    # Auto-required via bundler.
Assume 'mocha' has been required when requiring 'mocha/setup'

So no need to require 'mocha/version' in both files.

Note that 'mocha' is likely to have been auto-required if you are using it
within a bundler `Gemfile`, although be careful to remove any `require: false`
option that may have been on the gem declaration for earlier versions of Mocha.
Update release notes and bump version to 1.0.0.alpha.