Handle "internet explorer" as remote driver

This change allows the user to specify "internet explorer" (with a
space) as the browser with the remote web driver.

The motivation for this is the Sauce OnDemand Jenkins plugin:
https://wiki.jenkins-ci.org/display/JENKINS/Sauce+OnDemand+Plugin

This plugin sets various environment variables that relate to
Selenium, including:

 * SELENIUM_PLATFORM
 * SELENIUM_BROWSER
 * SELENIUM_VERSION

For the most part, you can pass these directly to splinter.Browser:

	url = ...
	browser = Browser(drive_name="remote",
					  url=url,
					  name="My test",
					  browser=os.environ['SELENIUM_BROWSER'],
					  platform=os.environ['SELENIUM_PLATFORM'],
					  version=os.environ['SELENIUM_VERSION'])

The problem is that, for internet explorer, this plugin sets

	SELENIUM_BROWSER='internet explorer'