Add a custom error message argument for missing schema nodes

missing_msg lets the developer specify a custom message in place of the
default "Required" if the node is missing.
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prep for 1.0b1
Merge pull request #123 from veeti/missing_msg

Add a custom error message argument for missing schema nodes
fix resource warnings
Un-break wrapping of callable instances as 'colander.deferred'.

Fixes #141.
all subclasses of SchemaNode need a ``typ``

also the docs are confusing with you do:
    from colander import SchemaNode
and then in the same code block you use ``colander.Range``, so I
formalized everything to use ``colander.*`` syntax.
Merge pull request #1 from Pylons/master

latest from upstream