This change introduces the use of system keyring to store repository
passwords when one is available. Passwords are added to and removed from
the keyring using the config command.

Resolves: #210

This change ensures that short hashes are taken into consideration when
evaluating if the required version of the dependency is already
installed.

The previous implementation only skipped update of a git
dependencies if the revision specified in the pyproject.toml matched
the full hash of the installed package.

Resolves: #1157

* Add support for custom urls in metadata

* Add documentation for custom urls

* Add support for per format packages

* Add documentation for per-format packages

* Add support for specifying dependency source

* Make installation from forced repositories possible

* Add possibility to declare a custom repository as the default

* Improve handling of sources in pools

* Fallback on the default repository when install via pip

* Add support for declaring repositories as secondary

* Update documentation

`poetry build` creates a whl file using `os.mkstemp`. The file is
created with very restrictive permissions. This commit ensures these are
normalized before the build starts.

Partly addresses https://github.com/sdispater/poetry/issues/671.

- Fix non-sha256 hashes retrieval
- Fix resolution of packages with Python 3 only wheel

* use symlinks in created venv by default on non-windows platforms. Issue #779

* Fixed test for deployments without venv module available.