Add encoding to Python source

I get the error below with ST 2.0.2 build 2221. The PR fixes it by declaring `Sourcegraph.py` as utf-8 encoded (which it is, as it has unicode literals).

````
Writing file /Users/nwhite/Library/Application Support/Sublime Text 2/Packages/Sourcegraph/Sourcegraph.py with encoding UTF-8
Reloading plugin /Users/nwhite/Library/Application Support/Sublime Text 2/Packages/Sourcegraph/Sourcegraph.py
Traceback (most recent call last):
  File "./sublime_plugin.py", line 62, in reload_plugin
  File "./Sourcegraph.py", line 114
SyntaxError: Non-ASCII character '\xe2' in file ./Sourcegraph.py on line 114, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details
````