Add files in package.json

[SimenB]

To avoid unnecessary files (tests and build-scripts). Renamed license to get npm to automatically pick it up. Can be added to the array if you want to keep old name.

Before:

11461 jquery-placeholder-2.1.3.tgz
-rw-rw-r-- 1001/1001       998 2015-09-29 14:08 package/package.json
-rw-rw-r-- 1001/1001        41 2015-09-29 14:08 package/.npmignore
-rw-rw-r-- 1001/1001      4298 2015-09-29 14:09 package/README.md
-rw-rw-r-- 1001/1001       888 2015-09-29 14:08 package/Gruntfile.js
-rw-rw-r-- 1001/1001      9126 2015-09-29 14:08 package/jquery.placeholder.js
-rw-rw-r-- 1001/1001      3187 2015-09-29 14:08 package/jquery.placeholder.min.js
-rw-rw-r-- 1001/1001       473 2015-09-29 14:08 package/bower.json
-rw-rw-r-- 1001/1001      4195 2015-09-29 14:08 package/index.html
-rw-rw-r-- 1001/1001      1075 2015-09-29 14:08 package/LICENSE-MIT.txt
-rw-rw-r-- 1001/1001       245 2015-09-29 14:08 package/CONTRIBUTING.md
-rw-rw-r-- 1001/1001      4303 2015-09-29 14:08 package/jquery.placeholder.min.js.map
-rw-rw-r-- 1001/1001         8 2015-09-29 14:08 package/.gitattributes
-rw-rw-r-- 1001/1001      4944 2015-09-29 14:08 package/tests/tests.js
-rw-rw-r-- 1001/1001      1787 2015-09-29 14:08 package/tests/index.html

After:

5126 jquery-placeholder-2.1.3.tgz
-rw-rw-r-- 1001/1001      1038 2015-09-29 14:10 package/package.json
-rw-rw-r-- 1001/1001      4298 2015-09-29 14:09 package/README.md
-rw-rw-r-- 1001/1001      9126 2015-09-29 14:08 package/jquery.placeholder.js
-rw-rw-r-- 1001/1001      1075 2015-09-29 14:08 package/LICENSE.txt