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.
update the statement of testing if a handler is a list or tuple
Add hooks for HTTPClient and HTTPConnection customization.
Document tornado.concurrent.Future.

All internal links now point there instead of to concurrent.future.Futures.
Fix draft76 websocket protocol; fix websocket demo for python 3.
digest authentication
Fix error handling with the combination of @asynchronous and @gen.coroutine.

Also make self.finish() optional for coroutines since we can finish
if the future resolves successfully.
Delete HTTPConnection._close_callback in between requests.

Previously the close callback would cause a reference to the previous
RequestHandler to be retained while waiting for the next request.
Bump version to 3.0b2
Fix options.parse_config_file on Python 3.

Add a test for this function.

Closes #702.
Update pypi description field.
Replace the outdated README.md with a lightly-edited version of docs/index.rst
Fix rst markup in readme