Fixed bugs relate to prevent copying to itself.

[iwege]

I try to fix the issue #60, If I missed something , please let me know.

This PR refactoring the startsWith function and try to handle more situations. I add more testcases to test the different situations. * Some testcases can't used in Windows* :

1. Copy to sub-folder
   1. src to src/out
2. Copy to itself
   1. src to src
3. Copy to the folder starts with src
   1. src to src_out
4. Copy to the symlink folder
   1. src to src-symlink
5. Copy the file to itself
   1. src/a to src/a
6. More complex situations:
   1. src to src/src_out
   2. src to src_out/src

And I add mkdirp to devDependencies to create the directory easily.