Get rid of a couple more context object references, fix some markup and clarify what happens when a generator context function swallows an exception.

Finish bringing SVN into line with latest version of PEP 343 by getting rid of all remaining references to context objects that I could find. Without a __context__() method context objects no longer exist. Also get test_with working again, and adopt a suggestion from Neal for decimal.Context.get_manager()

discussion.
There are two places of documentation that still mention __context__:
Doc/lib/libstdtypes.tex -- I wasn't quite sure how to rewrite that without
spending a whole lot of time thinking about it; and whatsnew, which Andrew
usually likes to change himself.

terminology in the alpha 1 documentation.

 - "context manager" reverts to its alpha 1 definition
 - the term "context specifier" goes away entirely
 - contextlib.GeneratorContextManager is renamed GeneratorContext

There are still a number of changes relative to alpha 1:

  - the expression in the with statement is explicitly called the
    "context expression" in the language reference
  - the terms 'with statement context', 'context object' or 'with
    statement context' are used in several places instead of a bare
    'context'. The aim of this is to avoid ambiguity in relation to the
    runtime context set up when the block is executed, and the context
    objects that already exist in various application domains (such as
    decimal.Context)
  - contextlib.contextmanager is renamed to contextfactory
    This best reflects the nature of the function resulting from the
    use of that decorator
  - decimal.ContextManager is renamed to WithStatementContext
    Simple dropping the 'Manager' part wasn't possible due to the
    fact that decimal.Context already exists and means something
    different. WithStatementContext is ugly but workable.

A technically unrelated change snuck into this commit:
contextlib.closing now avoids the overhead of creating a
generator, since it's trivial to implement that particular
context manager directly.