changed around filtering so you can just say "decode.utf8" or "decode.<whatever>" for generic expression decoding

  immediately catch any encoding issues and ensure internal unicode
  representation.

  [ticket:16]
- cleanup in unit tests

- added "default_filters" argument to Template, TemplateLookup.  applies only to expressions,
gets prepended to "expression_filter" arg from <%page>.  defaults to ["unicode"], so that
all expressions get stringified into u'' by default (this is what Mako already does).
By setting to [], expressions are passed through raw.
- added "imports" argument to Template, TemplateLookup.  so you can predefine a list of
import statements at the top of the template.  can be used in conjunction with
default_filters.

magic encoding comment

file
- added modulename_callable to TemplateLookup to allow a function to determine module filenames
(takes filename, uri arguments).  used for [ticket:14]

with a template that indicates a non-standard encoding, are expanded into backslash-escaped
glyphs before being AST parsed [ticket:11]

this is implemented by changing the "topleveldefs" and "closuredefs" collections
from a Set to a dictionary.  a unit test was added with alternate set-ordering
as the original issue only appeared on linux to start.
- "backslash" -> "slash" in syntax doc

occurs before filtering
- removed encoding() filter - it conflicts with the fact that unicode conversion has to
occur on the value first before being sent to the filter.  recommended way for encoded
strings is to just say unicode(x, encoding='whatever')

handling for the given expression.
usage is like:  ${data | encoding('utf-8', errors='strict')}

further try/except AST fixes, cleanup of filter unit tests, made __locals propigation in codegen slightly simpler/faster

more doc construction

by the template but otherwise not affected

implemented "args" for <%page> tag.  still has a default
"**_extra_pageargs" catchall for now...

- more docs

occurs in a directory hierarchy resembling uri scheme

cache function optional