`filter` parameter `fs.copy` and `fs.copySync`

`filter` can be a function or a regular expression

for filter:function, `filter(src)` must be true for copy
for filter:RegExp, `filter.test(src)` must be true for copy
merge 0.7.1
`filter` is optional for `fs.copy()`
Merge pull request #36 from Srirangan/master

`filter` parameter `fs.copy` and `fs.copySync`