Add $offset param to uv_fs_read() to allow seeking on open handles

This change modifies the uv_fs_read() function signature slightly by adding
a new $offset parameter. This allows the same file handle to be reused for
multiple reads. Previously the only way to access a section of a file that
had already been read was to open a new handle.

Old:
uv_fs_read(resoruce $loop, zval $fd, long $length, callable $callback)

New:
uv_fs_read(resoruce $loop, zval $fd, long $offest, long $length, callable $callback)

This change represents a minor BC break for existing code using uv_fs_read().
merging offset changes from PR #60