getActiveEditor returns undefined if no window is open

* lib/symbols-view.coffee:45 attempts to grab the current editor
  and then feed the cursorBufferPosition into var previous.
  If there is no current editor open, an undefined error is thrown
  on getCursorBufferPosition when trying to assign var previous.

* Add an if statement to check the editor before calling
  getCursorBufferPosition() on an undefined object.
Merge pull request #31 from PhilCoggins/getActiveEditor_returns_undefined

getActiveEditor returns undefined if no window is open
Call afterTagOpen at end of openTag
Destructure tag to position
Prepare 0.52.0 release
Add jumping back for file-view and project-view

Current jumping back only support go-to-declaration(cmd-option-down).
Now it can be used when jumping by file-view(cmd-r) or
project-view(cmd-shift-r)