Yay

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- An earlier version added validation to stop you replacing lists with mappings
or vice versa. However the implementation didn't consider that you might
extend a mapping that comes from a lookup.

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- Fix filter node. It was previously unaware of its parent when expanded and
this stopped its children from being able to look up values in the global
namespace.

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- Fix setting GPG_TTY.

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- Capture GPG failures and throw exceptions. Unfortunately without talking to
GPG over the status fd we can't get much information about what went wrong.

- Run in batch mode. This stops GPG from ignoring the stdout/stderr redirection
but means you now *MUST* use a GPG agent as it can no longer prompt for a
passphrase.

- GPG_TTY is set - this means gpg-agent can actually prompt now.

- Fix ``Config.add`` when using nested dictionaries.

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- Add new lazy lookup method to the root config object. This is useful if you
are building a lazy iterator over a dictionary key::

def example_iterator(self, expression):
try:
for item in expression.expand():
resolved = item.resolve()
create_obj_from_settings(resolved)
except CreateObjectError:
You can get line and column from the item object!
except yay.errors.Error:
Ideal place to wrap Yay errors in your apps exceptions

example_iterator(config.lookup("somekey"))

You can see here that the API lets you defer any messy "key not found"
exceptions until the iterator needs to access they key - and the iterator
already has apropriate exception handling.

- Add additional test cases for backwards compatibility

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- Code paths are now decorated with tracing context. This ``__context__``
metadata gives a sense of what yay was doing when it went off the handle.