Self signed certs pinning

Fix #1061 

<del>Note: XML-RPC client is still using a "trust all" solution when making connection on self hosted blogs with SSL enabled. I got issues on shared hosting when SNI was enabled - Seems that apache library included in android does have issues with SNI. I'm working on a solution, but may require time.</del> fixed in https://github.com/wordpress-mobile/WordPress-Android/commit/3475733badb05b0511f44bb9f60e4510545ce400