the required file from remote url should be saved to given path.

for example,
```python
require.file(path='/tmp/libgit2-0.17.0.tar.gz',
             url='https://github.com/libgit2/libgit2/archive/v0.17.0.tar.gz',
             md5='8fa3a08dc28ecec4280b8677741eef3d')
```
file should be saved to `/tmp/libgit2-0.17.0.tar.gz` instead of `v0.17.0.tar.gz` in current working directory