Choose a completer such as SourceKitten based on the given path.

[mitsuse]

This pull-request for preparation of SourceKittenDaemon support. The main modifications are as follow:

• Separate codes for invoking sourcekitten from the other part.
• Deoplete-swift chooses a completer such as SourceKitten class by calling __choose_completer with the path of temporary file, which contains the contents of the current buffer.

For SourceKittenDaemon support, SourceKittenDaemon counterpart of completer will be required. It must implement the same interface of SourceKitten's complete method.

I assume that Source manages pairs of the port number of a daemon instance and the path of .xcodeproj and __choose_completer generate SourceKittenDaemon completer by using them.

This pull-request also contains some refactoring and refinement.Please read the commit comments. Sorry for conflicting with some part of #4.

[smallfx]

@mitsuse this looks great! i'm gonna merge it in and start working on the SourceKittenDaemon Source class.