Organize `require`s on import

This commit takes a big step towards readiness. Instead of blindly
appending a `var foo = require('');` line at the top, the plugin will
now organize all imports on execution. It does that by reading all
current imports (assuming beginning of file), mixes in the new import,
sorts them, then writes them back to the buffer.

Also included in this commit are a few messaging improvements.
Prefix messages with "[import-js]"

This will make it easier to know where the messages are coming from.
Add support for configurable lookup paths

In a large project you often have multiple places where js files may
be located. In a project I'm working on right now, we have the following
paths setup in our webpack.config.js:

  __dirname + '/app/assets/javascripts',
  'vendor/assets/bower_components',
  'node_modules',
  'spec/javascripts'

I'm adding support for a .importjs config file. This file should be in
YAML format, and the only configuration supported right now is
`lookup_paths`, an array of paths to look for js files in on importing.

The .importjs file should be placed in the root of your project.
Add <leader>j mapping

This will simplify invoking the import function. I stole the definition
from
https://github.com/wincent/command-t/blob/master/plugin/command-t.vim
Add README