Add a method for generating the tabkeys for sysdb.

The design of database has been changed in commit 4fa966bc0651500bec38061ce4b33e22a15407dd.
This method helps update the database by altering the db and generating
the tabkeys from m0,m1,m2,m3,m4 in the old schema.

I don't know whether there is any existing function here. I just can't find one. I am using the Boshiamy Chinese input method. After I upgraded from Fedora 18 to Fedora 20, ibus-table keeps crashing because of this exception: 'tabsqlitedb.py:781:select_words:sqlite3.OperationalError: no such column: tabkeys'. Therefore I wrote this function. 

But I don't really know in which function should this method be called. I tried to call it in tabsqlitedb.__init__(), but you need the root permission to modify the sysdb file. Another way is to wrap it into a command line tool to allow the user upgrade the db by himself/herself, but I don't know where to put it either. So maybe please accept this pull request and help me finish it, or tell me what can I do. Thanks!