Quickhttp

- Removed support for Python 3.6.
- Removed `DirectoryHTTPRequestHandler` which was only needed for Python 3.6.

- Changed module organization of package code to be more clear and explicit:
- Changed name of `quickhttp.core` to `quickhttp.http_server`.
- Changed name of `quickhttp.quickhttp` to `quickhttp.cli`.
- Added new `quickhttp.exceptions` module for all package custom exceptions:
- Added new base exception class `QuickhttpException` that all other exceptions from this package subclass.
- Added new exception class `InvalidSearchTypeError`.
- Changed name of `NoAvailablePortFound` to `NoAvailablePortFoundError`. It is now a subclass of `QuickhttpException`.

- Changed `--time` option name to `--timeout`.
- Changed timeout time to be from last request instead of from server startup.

- Initial release. :sparkles: