Fix for installing llnode as a global

[No9]

When npm explore is ran with a -g option the default folder is set to npm.globalDir
which is the root $GLOBAL/lib/node_modules folder containing npm.
But when it's ran in local mode it uses path.resolve(npm.prefix, 'node_modules')
See https://github.com/npm/npm/blob/latest/lib/npm.js#L395
This is causing the resolution of node-gyp to fail in global mode.

This patch tests the npm environment variable npm_config_global to see if it's a global install and appends the required sub directories to get it to work.

Tested on Ubuntu 16.04 with

$ git clone https://github.com/no9/llnode
$ cd llnode
$ npm install -g 

Fixes: #77

[hhellyer]

The fix looks good to me, I've tested locally and both local and global installs now work.

[rnchamberlain]

LGTM - tested on Ubuntu 16.04. It's very useful to have install -g working for this - thanks!

[No9]

Thanks for the reviews - nits are resolved and PR updated.

[indutny]

Landed in fb24599, thank you! Sorry for delay.

[hhellyer]

@indutny - Since this is a fix to npm installation I'll update the version to 1.5.1 and publish a new version of the npm on Monday, unless you object!
(The brew installation probably doesn't need an update for this.)

[rnchamberlain]

@indutny @hhellyer could we land the README fixes as well, then publish? #85 and #86

[hhellyer]

@rnchamberlain - Yep, I'll do that today.