handle missing content-length header, rename folder simple_httpfs to httpfs

make consistent with module name = httpfs

also, change imports to make it portable = run without install

```diff
diff --git a/httpfs/__init__.py b/httpfs/__init__.py
index df9fe47..550da80 100644
--- a/httpfs/__init__.py
+++ b/httpfs/__init__.py
@@ -1,3 +1,3 @@
-from .httpfs import HttpFs
+from httpfs import HttpFs

__version__ = "0.4.12"
diff --git a/httpfs/__main__.py b/httpfs/__main__.py
index bb713b2..41543ab 100644
--- a/httpfs/__main__.py
+++ b/httpfs/__main__.py
@@ -5,7 +5,7 @@ import sys

from fuse import FUSE

-from .httpfs import HttpFs
+from __init__ import HttpFs


def main():
```

### handle missing content-length header

work in progress: https://github.com/milahu/fuse-httpfs/tree/wip-stash

* rename simple_httpfs to httpfs
* cache file_data on first get request in HttpFs.getSize

edit: downloading files only for the file size is a terrible waste of bandwidth.
better solution: show file as symlink -> no need for file size
download only when symlink is resolved in `readlink` syscall

some http servers dont support range requests, so `range 0-1` has no effect