Some extension of plugin

HI, just wanted to have control of what should be compressed depending on size and wanted to have an option to NOT append .gz extension (usually CDN can respond with correct headers independently of extension).

Result - 2 options can be passed to plugin:
* append <true> - ability to choose whenever you want to append .gz extension. Can be set to ```true``` or ```false```.
* threshold <false> - ability to set minimum size of compressible file (hard limit to 150 bytes). Reason is that some compression overhead with small files. Can be set to ```String|Number|Boolean``` - details below.

Plugin handles ```Buffer```s and ```Stream```s.

Actually tried to make ```threshold``` option pretty handy - it can receive ```String``` (parsed by ```bytes```, hard limited in 150), ```Number``` (exact size in bytes, hard limited in 150), ```Boolean``` (false - compress independently on size, true - resulting hard limit - 150).

Also replaced event-stream with through2 - reason is that ev built with Streams1 in mind and during extending of your plugin, I had couple of problems because of incapability of ev with Streams2.

Also rewritten tests - including originals with addition of test suit for extended capabilities. Now for testing you can use ```npm test```/```gulp```. Also added as dev-dependency gulp with little zoo of plugins allowing to livecode while testing.

Also added ```example``` folder demonstrating few usage scenarios.