remove the last '/' of $url
add screenshot feature.

$screenshot = $driver->takeScreenshot($save_as);

$driver->takeScreenshot() return the png image.
If you want to save the screenshot to somewhere, pass the path of as
$save_as. If $save_as is null, then it won't be save.
refactor and simplify

It is bad to rewrite execute() and pass the $session_id and $executor
everytime when constructing a new WebDriver class. It increases the
friction when building the api. This diff does not break the interface
of all methods. Just Simplify the constructor and reuse the execute().

Changes:
- remove execute() in each WebDriverXXXX
- use $driver to construct WebDriverXXXX
add 'sendFile' to WebDriverCommandExecutor

This is a secret command... not yet included in the offical document.
Also reorder the commands.
Support Packagist (Composer)

This might break thing because the source files are moved to /lib but
it should be beneficial in the long term.
fix autoload in compose.json
Update README.md
FIX WebDriverWait constructor: Type hints can not be used with scalar types
Merge pull request #65 from Mordred/master

Type hint in WebDriverWait constructor