Add more convenience math functions

[mavenlin]

Many functions are missing from caffe.
For example

1. elementwise abs
2. sum of matrix, col sum row sum and total sum. (though can be done by gemv, it might be more convenient to provide a wrapper, multipliers can be allocated at run time and store in a global map).
3. non-in-place scaling, currently it is in place.
4. elementwise sign

A large proportion of the commonly needed function are elementwise operations.
Will we consider bringing in additional dependencies? e.g. thrust

[sergeyk]

Great suggestions! Anyone who sends PRs (into dev, with test) referencing this issue will be promptly merged.

[kloudkl]

I used sum of abs in #113, element wise sign in #113 and #141, and hamming distance in #141. Non-in-place scaling is a combination of copying and in-place scaling. I am trying to figure out the others and a PR is on the way.

[shelhamer]

Closing, as this will be solved by #159. Thanks for the suggestion @mavenlin and commits @kloudkl.