(license): re-license under Apache and add a LICENSE file

- so Physijs is MIT and physijs-browserify is ISC...
  - I think I put it back to MIT to stay consistent with Physijs
    when I originally forked this a few years back
    - and probably haven't noticed since as I never even added a
      LICENSE file...

- use Apache as I prefer it over MIT and to stay consistent across all
  of my libraries
- they are similar enough so don't think this will be a huge issue to
  small userbase of this, but could be considered a breaking change
  - also open to changing back if discussion reveals this as a problem

LICENSE

@@ -0,0 +1,13 @@
+Copyright (c) 2019 Anton Gilgur
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.

package.json

@@ -12,7 +12,7 @@
     "browserify-worker-stub.js"
   ],
   "author": "Anton Gilgur",
-  "license": "MIT",
+  "license": "Apache-2.0",
   "homepage": "https://github.com/agilgur5/physijs-webpack",
   "repository": {
     "type": "git",