Possible workaround for #270

Hello!

I try to move from SeleniumLibrary to Selenium2Library in tests at my work. The biggest problem I've faced on this way is an impossibility to select some dialogs created by our web system. When a dialog opens, words `Get Window Titles`, `Get Window Identifiers` and `Get Window Names` causes an internal `TypeError`. This bug is 100% reproducible on some dialogs when running tests with Firefox 24.2.0 (I did not test on other browsers).

I've investigated that this bug appears when `WebDriver` returns `WebElement` as window id. It looks like this (just a piece of debug log, when I've added logging into `get_current_window_info`):

>
...
WARN	[None, u'', u'YC4EmApxD9_Opms.Very Temp_\u041e\u0431\u0437\u0432\u043e\u043d', None]
WARN	[None, u'', u'\u0424\u043e\u0440\u043c\u0430_ix5EOTKF', None]
WARN	[\<selenium.webdriver.remote.webelement.WebElement object at 0x2a90390>, u'{"pos":{"x1":427,"y1":295,"w":0,"x2":426,"h":0,"y2":294},"win":{"width":429,"height":377}}', u'\u0414\u043e\u0431\u0430\u0432\u043b\u0435\u043d\u0438\u0435 \u0430\u0442\u0440\u0438\u0431\u0443\u0442\u0430 \u0438\u0437 \u043a\u043b\u0438\u0435\u043d\u0442\u0430', None]

The last record appears right before the fail.

So, I've added some code into this function in order to handle `WebElement` correctly. Now none of our dialogs produces this error, and all library unit tests still pass.