Move build settings to user settings. Add js_lint_folder command.

I really like your plugin, but the problem about using a `.sublime-build` is that though a user *can* edit the build settings in the `Packages/JSLint/JSLint.sublime-build` file, but those changes **maybe overwritten** any time the plugin is updated.

So:
* I implemented a little helper command `js_lint_exec`, which basically runs a *custom build* reading the settings merged from the default `Packages/JSLint/JSLint.sublime-settings` and custom `Packages/JSLint/JSLint.sublime-settings`
* This way we can still get the *Jump to errors using F4 / Shift+F4* functionality for free.
* Plus, **using node-jslint** we can now **lint through folders** using the filename filter defined in the settings and again *Jump to errors using F4 / Shift+F4* **even in different files** easily.
* In the end I kind of rewrote the whole plugin, but the main idea is still to leverage Sublime's in-built functionality of running terminal-commands like a build-script.
* I did keep the default jslint options (like "--sloppy"...) and the keymap as you defined it to make the transition easy.
* Also, I updated the README.md. In particular, I added a section on the problem which was discussed in https://github.com/darrenderidder/Sublime-JSLint/issues/8.

I do realize that this is basically a whole new plugin (codewise at least), but I don't want to add another sublimetext-jslint plugin to the package control repo, since there are already two of them. So, if you'd like to pull this stuff into your repo, please go ahead.