Intro nb added
clean up and regression
preprocessor example
Regression nb
lars section
some more additions
revamp structure
refine lars and intro
Modular API for RF and minor fixes
loss functions added for Gradient Boosting
extend svm nb
Minor refactoring in:
    * all the StructuredModels
    * all the StructedOutputMachines
to provide support for sparse joint feature vector
Modified all the StructuredOutputModels to include
get_sparse_joint_feature_vector() method.
Implemented CHashedMultilabelModel class to faciliate feature hashing in
multilabel scenario.
Added unit-test for CHashedMultilabelModel
some cleanups
add_to_dense() method added in SGSparseVector for adding a dense vector
Minor refactorings and changes in:
    * all the StructuredOutputModels
    * all the StructuredOuputMachines
to remove redundant methods and intializations
fix abstract of log-det notebook
Minor bug fixes in HashedMultilabelModel