Add support for required file validation

[zulfajuniadi]

This commit adds support to validate required file inputs.

[coveralls]

Coverage remained the same at 95.62% when pulling 1d0398b on zulfajuniadi:master into e618205 on chemerisuk:master.

[chemerisuk]

@zulfajuniadi please don't modify files from the dist folder - do it in the file from src.

Second issue with your code - it doesn't work in IE8 (http://stackoverflow.com/questions/12830058/ie8-input-type-file-get-files). But you can check value property instead, so the logic is the same as for general case. In other words, just leave the change on 12th line and it should work.