Add 500px and Searchcode engines
Allow to search for images on 500px. It doesn't use the official API, but the page result. Less stable, but less API key to possess...

Two engines were necessary for Searchcode because there are to search mode : search for documentation or search for code example. Both use open APIs.
[fix] fix language support in photon engine
Merge pull request #158 from Cqoicebordel/Moar-Engines

Add 500px and Searchcode engines
Merge pull request #159 from pointhi/photon_fix

[fix] fix language support in photon engine
[enh] engine shortcut validation