Add support for import manipulation with importmagic

* Add a class that bridges to an importmagic symbol index
* Add RPC calls to fixup imports, or add a single import
* Add Elisp functions to use these features.

  (elpy-importmagic-add-import): Query user for a name to import and
  the module to import it from.  A list of candidate modules is fetched
  from the symbol index.  The import block is updated automatically.

  (elpy-importmagic-fixup): Fixup imports (remove unneeded imports and
  add imports for unresolved identifiers).  [This feature seems to be
  a bit broken in importmagic.]

Related to #330.