Update ctagsplugin.py

1. Modifies class RebuildTags.build_ctags:
       Generate a new file .tag_cmp, which contains all vocabulary of the project

2. Modify class CTagsAutoComplete:
       Improve auto competion across project by using .tag_cmp file
       Now on_query_completions uses open instead of popen
(Verified in fedora 20, ST3)