Implement additional error handling in utils.import_object function
Merge remote-tracking branch 'upstream/master' into import-object
import_object will raise standard ImportError even in case of Value/Attribute error
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.
correctly handle empty POST parameters
Mv README so github web interface recognize its markup language
Add a helpful __repr__ implementation to URLSpec.
Allow add_handlers() to append to existing hosts.

This change removes the restriction that all handlers for a given host
pattern be registered in a single call to add_handlers().
Fix a bug with IOStream.read_until_close with a streaming_callback.

If the underlying socket was already closed the buffered data
would be passed to the final callback rather than the streaming callback.
Merge pull request #624 from pablopda/master

Move README so github web interface recognize its markup language
Merge pull request #627 from jparise/urlspec-repr

Add a helpful __repr__ implementation to URLSpec.
Fix TwitterMixin on Python 3.

Also add tests, and add get_auth_http_client method to all auth mixins.

Closes #634.
Ensure that add_callback fails cleanly if called while IOLoop is closing.

Previously there would (sometimes) be an exception when writing to the
waker pipe.

Closes #635.
Fix a memory leak in stack_context.

The old_contexts reference in StackContexts could maintain a chain of
old irrelevant contexts, so clear it once it's no longer needed.
This was mainly a problem in gen.engine, where additional contexts
would accumulate in memory (but not on the stack) for each asynchronous
operation.

Also clear the deactivate_stack_context in gen.Runner to allow
the StackContext to be garbage-collected sooner.
Fix If-None-Match support for StaticFileHandler.

No longer set Cache-Control: public on static files; redbot.org says
it generally doesn't do anything useful.
Make header_callback behavior consistent across both HTTP clients.

SimpleAsyncHTTPClient now sends the first and last lines to the callback,
matching CurlAsyncHTTPClient's behavior.

All secondary callbacks are now wrapped for stack_context.

Closes #637.