Move history management feature to new file

One feature par file is Gherkin (Cucumber) convention
https://github.com/cucumber/cucumber/wiki/Gherkin

It is also easy to manage features this way.
Remove @failing tags from already fixed test cases
Fix: blank buffer appears when opening a file

When opening a file under version controlled system, e2wm
shows *e2wm:blank* buffer in the main window.  This happens
because kill-buffer-hook is called when vc-follow-link kills
newly opened buffer (it tries to follow symbolic link).

Inside of e2wm:kill-buffer-hook, WINS is nil because the
buffer is not shown yet.  But BUFFERS is also nil because
this buffer is not registered in history.  Therefore,
ignoring both cases in COND is the right choice.  However,
it executed the second clause because MAIN-WNAME is non-nil.
This clause must be executed only when WINS has some windows
in it.  Thus, the check for WIN is added.
Remove @failing tag from the test cases fixed by this PR
Add e2wm:$pst-focus slot to save focused window name
Treat minibuffer case in e2wm:pst-copy-instance
Restore focused window in e2wm:override-restore-window-cfg
Remove @failing tag from the test case fixed by this PR
Add Travis CI setup
Run test-e2wm-pst-class.el in Travis CI
Merge pull request #48 from tkf/organise-ecukes

Organize ecukes
Merge pull request #50 from tkf/travis

Travis CI
Merge pull request #46 from tkf/fix-vc-blank-buffer

Fix: blank buffer appears when opening a file
Merge pull request #49 from tkf/pst-focus

Fix changing focused window after opening and then closing *Completions* window