Find nested packages with excluded parent

`find_packages(exclude=['pkg'])` should still find and include the
`pkg.subpkg` package.

Fixes #808

Fix misspellings in changelog.

Note find_packages backwards incompatible change

Also add a test for the new behaviour.

Do not search excluded directories for packages

Previously, PackageFinder.find would search the whole directory tree
looking for packages, then remove excluded packages from this list. This
made building a package very slow under some circumstances where the
file tree was large.

This change stops PackageFinder.find from descending in to directories
that will never be included.

Add missing whitespace.

Upgrade pyparsing to version 2.1.9

In test command, add installed eggs to PYTHONPATH when invoking tests so that subprocesses will also have the dependencies available. Fixes #794.

pytest: ignore all .* directories

Don't duplicate error case in package_index

easy_install has code to handle parsing a requirement, catching the
ValueError and then raising a DistUtilsError. This code was entirely
duplicated in package_index, so I've slightly refactored to remove
the duplication.