Frank-Wolfe SOSVM solver

This patch contains the following changes:
- Implemented the Frank-Wolfe batch algorithm for SOSVM. 
- Corrected an error in dual objective computation in SOSVMHelper.
- Refined documentation of SOSVMHelper. 

Hi @iglesias and @tklein23, would you like to revise the code? Also, merge it for me when Travis feels happy about it? :)