* Typing syntax fixes

* Fix some function signatures that were messing with ellipsis
  for example, `fun(a, ...)` is not valid, but `fun(...)` is

* Dict value types - is that legal? (puzzle/provider.py)

* Ignore assigning instance to classes
  (singletons like Infinity in version/utils.py)

* Add basic mypy config

This configuration has all common settings like:

* strict Optional in functions that returns None

* ignore missing imports - this is usual thing

* Check code that has no typing annotation but uses typed classes/functions

This changes the behavior of version command to show the current version
of the project if no arguments are provided.

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.