Add ability to set download directory for webDrivers ( #167 )

[DSPaul]

Adds a new constructor for DriverManager that takes a path where the drivers should be downloaded to.
Current directory remains the default. Addresses #167.

[DSPaul]

Thank you for merging but in commit 9ce6c2f you changed the new constructor from public DriverManager(string downloadDirectory):this() to public DriverManager(string downloadDirectory) which causes the default constructor to no longer be called, which results in the DriverManager not having a BinaryService or a VariableService, which in turn causes it to crash when you try to use it.

There is no way to set these services outside the constructor because they are private, making the new constructor unusable and thus this entire PR. It is also the only thing you didn't revert in commit 75fa65e so maybe it's just an unintentional oversight. I can open a new PR if you want but the fix is pretty straightforward, just change it back. Unless there is a good reason to remove it that I'm not seeing, I would love to know.

[rosolko]

@DSPaul Yes, my fault, change it by mistake. Will fix it as soon as possible.