* Utility function to check if a callable has a given keyword argument

* Added unit tests for the has_arg function

* Replace uses of getargspec with the new has_arg function

Not changing keras.backend, because that gives ImportErrors due to
a circular import (conv_utils uses the backend, and is imported
before generic_utils in utils/__init__.py)

Not changing keras.utils.test_utils, because that change exposes
(what looks to me like) a latent bug

* Replace incorrect use of getargspec in test_utils.py

The previous code would always fail to detect the 'weights' argument.
Simply replacing getargspec would cause the tests for some of the legacy
layers to fail because the passed 'weights' argument is bad.

Instead, I have added a check for whether the passed `weights` array
is empty, this avoids tripping the bug.

* Replacing getargspec with has_arg in the backend modules

This requires reordering imports to avoid errors caused by
conv_utils trying to import the backend, the backend wanting to
import generic_utils, and utils/__init__.py listing conv_utils
before generic_utils.

* Removed getargspec from legacy wrapping function

Instead save the wrapped function in an attribute and call
getargspec on this attribute during documentation generation.

This reverts commit ced84c4b.

* Utility function to check if a callable has a given keyword argument

* Replace uses of getargspec with the new has_arg function

Not changing keras.backend, because that gives ImportErrors due to
a circular import (conv_utils uses the backend, and is imported
before generic_utils in utils/__init__.py)

Not changing keras.utils.test_utils, because that change exposes
(what looks to me like) a latent bug

* Added unit tests for the has_arg function

* Replace incorrect use of getargspec in test_utils.py

The previous code would always fail to detect the 'weights' argument.
Simply replacing getargspec would cause the tests for some of the legacy
layers to fail because the passed 'weights' argument is bad.

Instead, I have added a check for whether the passed `weights` array
is empty, this avoids tripping the bug.

* Replacing getargspec with has_arg in the backend modules

This requires reordering imports to avoid errors caused by
conv_utils trying to import the backend, the backend wanting to
import generic_utils, and utils/__init__.py listing conv_utils
before generic_utils.