cleanup toplevel namespace
Add basic top-level shortcuts
use "import scrapy" in templates
DOC use top-level shortcuts in docs
suggest scrapy.Selector in deprecation warnings
TST fix tests that became broken after adding top-level imports and switching to py.test.

py.test replaces AssertionError exception with its own subclass in an import hook.
For some reason when top-level shortcuts were added "except AssertionError" in
contracts.__init__ meant "except py.test AssertionError" while scrapy.exceptions.ContractFail
was still a subclass of a builtin AssertionError, so Contracts reported errors
instead of failures and this made tests in test_contracts.py fail.

For now - just disable fancy py.test asserts.
Merge top-level-shortcuts