Restler

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- Upgraded SQLAlchemy from 0.5.8 to 0.6.0.

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- Allow an arbitrary SQL filter to be passed via a new query parameter
named `where_clause`. This parameter can contain any SQL that would be
valid as part of a where clause. Note that if other filters and filter
params are passed along with `where_clause`, they will be ANDed together.

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- When converting strings to IDs for an Entity, call `convert_param` on the
ID (or its parts for a composite key) so it (or parts) will be converted to
the proper type(s).

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- Finally started adding unit tests!

- Added support for specifying fields to include and/or exclude via the
`fields` query parameter. This is done by prepending a "+" or "-" sign to
field names.

Previously, the options were to A) specify the exact list of fields or B)
not provide a list to use the default list. The new +/- options allow for
'default set plus extras' and 'default set minus unneeded fields'.

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- Ensure SQLAlchemy 0.5.x until 0.6 comes out of beta.

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- Allow clients to specify the base URL for redirects via a custom header--
X-Restler-Client-Request-URL. Previously, the URLs generated for redirects
were relative. These were being converted to absolute URLs in the Location
header of the redirect. Obviously, this wouldn't work for requests coming
from a different domain.

XXX: Is there a better way to do this, perhaps with standard headers?