Do not try to complete Python in comments.

Auto-completion in comments does not make all that much sense,
and Rope will get seriously confused there, trying to complete
any Python symbol even in the middle of a string. Elpy now
does not even try to complete anything if we are inside a
comment.

Fixes #155
elpy-rpc-traceback: New command.

This command prints the last traceback in the backend to a new
buffer, for debugging purposes.

Fixes #171
Run the flymake check function from "/".

In some configurations, it is possible to break flake8 because
it will import from the current directory (specifically, when
PYTHONPATH contains the empty element/current directory). Running
flake8 from / ensures that there are no accidental imports happening.

Fixes #169
Ensure yas-snippet-dirs is a list before adding to it.

That variable is allowed to be a string, in which case it is
tretead as a directory by yasnippet. We now make sure that it
is turned into a list before we add to it.

Fixes #176
Remove before/after save RPC calls.

These were used by Rope, but it seems Rope can happily live without
them. Elpy hasn't been using them since 1.2, so we now removed them
completely.

See #148