* 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.

# Conflicts:
#	CHANGELOG.md
#	poetry.lock
#	poetry/__version__.py
#	poetry/installation/installer.py
#	pyproject.toml
#	tests/console/conftest.py
#	tests/installation/test_installer.py
#	tests/masonry/builders/test_builder.py
#	tests/masonry/builders/test_wheel.py