Incorrect operation sequence in bzr-handling code

Proposed fix to #1180 (and maybe #932 & #1104)

* Changed `PipXmlrpcTransport` to pass headers at request scope
* Added summary line to the changelog

* Added a `with` block to handle `session` life-cycle. As commented
  in #1902

Fix #1657 - Explicitly close the requests session

Use b"" instead of b()

Upgrade CacheControl to 0.10.2

This fixes a reference cycle issue where pip would randomly hang
forever in some of the tests.

Hide output when installing pip

Use diff-cover to show the change in coverage

Show coverage information for tests

Run the unit tests before the integration tests

Use a regex to avoid hard coding version branches

This is a rebased final version of a proposed solution to fix
issues #932, #1104 & #1180. Following changes have been done:

* Implemented a new class `PipXmlrpcTransport` using a
  contained `PipSession` object.

* Modified the `pip/commands/search.py` to make use of the
  `PipXmlrpcTransport` class.

* Properly initialized options for testing `SearchCommand`:
  - Changed `options_mock` to an `options` object built from
    `parse_args`, to properly initialize default options.

Clarify requirement specifier format

req: Support --no-use-wheel flag in requirements.txt

Query installed requirements for extras

fixes issue where egg_info('requires.txt') does not exist
for packages installed from wheels.

Removed reduce from pip's compat module

It already has a consistent location across Py2/3.

Remove unnecessary imports, move some to six

Overwrite old scripts when installing from wheel.

Fix #1101 - Properly handle a <base> without a href