Fixing issues with install's --target option.

 1. Check for the existence of a directory before copying from temporary directory to final target. If it exists, warn the user.
 2. If the user specifies the --upgrade option and the directory exists, delete it and continue with installation.
 3. Adding tests for above cases.

This resolves #1489, #1710 completely and parts of #1833.
Fixing pep8 errors
Covering case of single-module packages.

Behavior is now to overwrite files or folders if --upgrade is specified, however, links will not be overwritten.
update requests to 2.4.1 and add certifi dependency
remove dist-info files as well
update cachecontrol to version 0.10.3
make it pass
kill ChunkedEncodingError
revert travis change
Dont fallback to sitewide imports
Merge pull request #2024 from kevinburke/requests-2.4.1-try3

Requests 2.4.1
Upgrade colorama to 0.3.2
Upgrade html5lib to 0.999
Merge pull request #2007 from jonparrott/develop

Fixing issues with install's --target option.
Update logging calls to the match the stdlib logging
Merge pull request #2025 from dstufft/upgrade-dependencies

Upgrade dependencies
Merge pull request #2026 from dstufft/update-logging-calls

Update logging calls to the match the stdlib logging
Clarify the requirements file format documentation - Thanks @thanatos
Merge pull request #2027 from dstufft/correctly-document-requirements

Clarify the requirements file format documentation - Thanks @thanatos
We're passing upgrade to PackageFinder.find_requirement so this won't happen