prioritize browser detection, remove os dependency

[DamonOehlman]

So this is a reasonably simple change, but one that should have a pretty big impact for people who have been hit with the code path which is returning a false positive on node detection (see #54).

It changes two things:

1. We do browser detection if at all possible (i.e. navigator is defined). This then falls back to detecting the node environment if that isn't available.

2. Removes the require('os') import for os detection and simply uses process.platform instead. The information won't be as detailed and it's possible that a user of this module may still want to opt to do their own check using the os module. Given that the majority use case is web browser detection though I think that makes sense. Should stop browserify bringing in any shims for the os module also (which is a good thing).

Because of point 2 though, I believe we should do a major version bump to 3.0.0 to ensure that no-one that is using detect-browser in an isomorphic way is caught out unexpectedly.

@5punk PTAL.

/cc @shengshiqi @markgoho-EDT

[DamonOehlman]

@5punk Hey mate - just wondering if you've had time to look at this. I'm keen to create a 3.0 version soon as there is another feature that has been requested that would definitely need a major version bump to be included.

[5punk]

I'll take a look at this today.
Sorry for being AWOL.

[5punk]

I would feel a little to see a unit test for this use case. Maybe in a future rewrite we can use webpack/rollup, typescript and jest/ava

[vstets]

Hello guys,

Please complete this PR - we are waiting on it.

[DamonOehlman]

@vstets The PR will be merged shortly. I'll add some comments on your issues / PRs shortly so we can action that also.