Use gecko/chromedriver from PATH

Helium ships with its own copies of gecko- and chromedriver. These binaries quickly become outdated. Previously, there was no easy way to "fix" Helium's webdrivers for your machine.

No longer. Now, Helium first attempts to use chromedriver and geckodriver from your PATH. This lets you install the versions of those drivers that work on your system.

The implementation is based on a PR by @TotallyNotChase. Thank you!

Improve support for dragging between `Point`s

See the new test test_drag_point.

Make it possible to supply FirefoxOptions to start_firefox

Thank you @ftnext for the PR!

Make it possible to supply ChromeOptions to start_chrome

Added an argument options to start_chrome(...). You can use it to supply the ChromeOptions when starting the browser. For example:

from selenium.webdriver import ChromeOptions
options = ChromeOptions()
options.add_argument('--start-maximized')
options.add_argument('--proxy-server=1.2.3.4:5678')
start_chrome(options=options)

Better handle ElementClickInterceptedException

Thank you Tim Powell (@tbp105) for the PR (#2)!

First open source release

v3.0.0

Update README.md