Yay

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- Fix PackageIndex to not warn() on missing package - we capture this
ourselves!

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- Line number and file information should be available for nodes that have been
cloned leading to much more useful error output.

- Index errors on Sequence nodes will be properly dealt with

- Treating a sequence as a mapping node will trigger an appropriate Yay error.

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- Allow python code using yay to set default configuration for openers

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- Fix regression in ``package://`` for existing users..

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- Introduce a lazily evaluated mechanism for expanding the search path::

.search:
- package://some.egg/
- http://raw.github.com/isotoma/yay/

Any subsequent ``.import`` statements will use the modified search path.

- If a package specified either as part of a .include or as part of a .search
is not available on sys.path then an attempt will be made to install it.

- Can pass config to opener backends using ``.config``. For example, you might
want to have the ``package://`` opener to use your internal package
repository::

.config:
openers:
packages:
index: https://my-python-mirror/simple/
username: joe
password: penguin55

You can use variable subsitution and to define the password in a GPG
encrypted yay file or to push the variable from application that is using
Yay.

- Yay openers support basic auth in the url. If used with .include then use of
secrets to concatenate to hide the password is suggsted::

 Define ${password} in a GPG armored file
.include: home://.credentials.yay.gpg

.search:
- https://username:${password}svn.yourcompany.org/svn/cookbook/trunk

- ``.include`` and ``.search`` may be a single scalar value rather than a list
if you desire.

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- Allow expressions to return an empty dictionary::

result: ${site.vhosts else {}}