Added multilabel reader in LibSVMFile

There are several issues need to be discussed:
(1) Use SGVector or SGSparseVector to store labels. Currently, it is SGVector but there should be no trouble to change it into SGVector.

(2) There is just one unified function for parsing single-label and multi-label files. However, I kept the olde API of reading single-label file. So the SGSparseVector labels are converted internally. To move this  converter outside, I guess many files dependent on the old API need to be changed. 

@tklein23 @vigsterkr 
For (1), if you all agree, I can use the unified type SGVector, since it is more compatible with @tklein23 requirement.

@vigsterkr
For(2), I need your help since I don't know how to put the converter outside and how many files need to be changed correspondingly. To be honest, I don't fully understand your design. But I think all the necessary code are already in LibSVMFile right now.