Add a formatSize method to NumberHelper

Let's introduce a method to format file sizes.

Examples:
* formatSize(0) returns '0 bytes'
* formatSize(1) returns '1 byte'
* formatSize(1000) returns '1,000 bytes'
* formatSize(1024) returns '1.00 KB'
* formatSize(2000000) returns '1.91 MB'
* formatSize(-5000) returns '-4.88 KB'
* formatSize('hello') returns 'hello'
Remove spurious php code
Merge pull request #1389 from mlocati/add-format-size

Add a formatSize method to NumberHelper