make the client more configurable

• add configuration AllowedContentTypes, the validating process is keyword search.
• add configuration AllowedFileExtensions, the file extension will get from url's path, the validating process is suffix check.
• add configuration InsecureSkipVerify, the default value is false. We use to force skip certification verify, but after adding this configuration, the default behavior would be validate certifications.

v0.2.0

Merge pull request #3 from leeyifei/master

v0.1

detect image size without request full image file