Add optional conjunction parameter to oxford

[timothysu]

A backwards compatible change to the oxford function to support custom conjunctions (rather than it always being 'and'). Also a change to the readme, as npm run install is now npm install.

items = ['apple', 'orange', 'banana', 'pear', 'pineapple']

// Pick a conjunction (defaults to 'and')
Humanize.oxford(items, null, null, "or")
// "apple, orange, banana, pear, or pineapple"

// Also works with a limit
Humanize.oxford(items, 3, null, "with")
// "apple, orange, banana, with 2 others"