Better support for browser windows

browser.windows. Now an object that lazily creates a Window
object for each window. You can access windows by index or by
window handle string. browser.window.current points to the
current window object. Simply assign a different window to current
to switch windows.

Window object. .prev/.next properties give you the previous and
next windows (wrapping around end of list). .is_current is a bool
indicating if this window is the currently active window. set
is_current to True to make it current. .close() closes this window
.close_others() closes all other open windows.

passing tests and basic documentation included.
Merge branch 'master' into window-orig
added notice that window interface not backwards compat
Merge pull request #310 from dgreisen-cfpb/window-orig

Better support for browser windows