Adding support to override, via static_url, whether the generated URLs
should include the version signature or not.
Introduce StaticFileHandler.(get_content|set_headers) which reduces the
complexity of the HTTP GET handler of static resources.
Fixed syntax error in the altered StaticFileHandler.get() method
Move retrieval of absolute path to separate method which is also in
charge of validation.
Cache the computed absolute path to the requested resource in
StaticFileHandler. The cache is cleared on_finish of each request
in order to ensure recomputation on each request.
Merge pull request #1 from facebook/master

merge master from facebook tornado
fixed bug when raise error in options
comparison to None should be 'if cond is not None'
Use 'get_version' to calculate static file hashes.
(incomplete) Honnoring Range request header
Add test for option redefinition error.
Fix bytes/str issues
Dereference the current YieldPoint as soon as it resolves.

These references could otherwise keep a chain of old references alive
(test code: https://groups.google.com/group/python-tornado/browse_thread/thread/37d3928817e4924d)
Merge branch 'branch3.0'
Set Accept-Range header, return 406 on invalid range
Tests for Range header
Handle negative byte ranges
Return HTTP 416 Range Not Satisfiable, not 406
Fixing errant paren, missing format value
Remove redundant call to find_named_blocks in template processing.

self._get_ancestors already returns self.file in the appropriate position.

Closes #791.