Introduce Keys.sequence

[tristanlabelle]

• Removes Keys.operator+ as it could be confused with "hold and press " in favor of an explicit Keys.sequence(...) function.
• Removes sendKeys overloads taking a Keys array. The array variant is supported by the protocol, but the spec says "The server should flatten the array items to a single string to be typed.", so let's rather encourage the use Keys.sequence. If a client really needs to send a key array, they can drop down to webDriver.send(Requests.SessionKeys(...)).
• Removes .combo, which was unused and assumed a certain ordering of modifiers that is better expressed in the client code.

Fixes #163

[jeffdav]

I think this is good. I think seq() would have been fine, but pixels are cheap these days. :)

[tristanlabelle]

@jeffdav I don't want to be the next person responsible for the fact that thousands of programmers have to deal with the fact that /usr/ is missing an e :P. Swift is a pretty verbose language already so this stays in style.