Added a method to retrieve the array of mobile browsers and changed '->userAgents' to '->browsers' for clarity.

Hi,
I needed to get the `->userAgents` array from outside of `Mobile_Detect.php` so I've added a method to retrieve it.

Then however, you end up with:
`$md->getUserAgent();`
`$md->getUserAgents();`
which in my opinion may be confusing.

There are two simple ways to differentiate between the user agent string that was taken from `HTTP_USER_AGENT` and the array of browser names and regex matches:
- either change `userAgent` to `userAgentString`
- or change `userAgents` to `browsers`

I've chosen the second options and here's the commit.