Use FeatureUnion to enhance feature detection

[Aurora0001]

• [x] Add FeatureUnion to Classifier class
• [x] Add Pastebin feature (to detect Pastebin, Gist, Pastie, etc.)
• [x] Add image feature (to detect imgur links)
• [x] Add post length feature

[Aurora0001]

Done as of 22c5012aa4f3f92a577df1667d3fd2a762fb9c00.