Exiv2

- Add a function [pyexiv2.set_log_level()](https://github.com/LeoHsiao1/pyexiv2/blob/master/docs/Tutorial.md#log) to handle logs.

- v2.0.0 is not compatible with previous versions, because the API has changed. See the new [tutorial](https://github.com/LeoHsiao1/pyexiv2/blob/dev/docs/Tutorial.md).
- Refactor the code to call the C++ API with pybind11 instead of ctypes.
- Remove the three ambiguous methods: `Image.read_all()`, `Image.modify_all()`, `Image.clear_all()`
- Add a method `Image.close()` to free the memory for storing image data.
- Add a parameter `encoding='utf-8'` to some methods.
- Converts stderr's log 'EXV_ERROR' to an exception. (Issue 8 )

- Could read and write the value of List type in XMP metadata.
For example:
py
>>> i.modify_xmp({"Xmp.dc.subject": ["flag1", "flag2", "flag3"]})
>>> i.read_xmp()["Xmp.dc.subject"]
['flag1', 'flag2', 'flag3']

When pyexiv2 processes it, if the raw value contains `', '` , the final value will be wrong.

- Add method `read_raw_xmp()`. Calling it will return the raw XMP metadata in XML format without mistake.

- Fixed an issue (3) about random crash.
- Updated description.

- Updated description.
- Correct the test cases. Ensure that the operation of reading metadata does not affect the image.

- no longer read `typeName`.
`typeName` is determined by Exiv2 for each metadata.
It was found that reading `typeName` of some metadata could go wrong, causing the program to crash. So, don't read it anymore.