lasso is not able to handle matrices instead of vectors

Here is an example that could be added to the test suite, for my problem I need to send matrices instead of vectors to the lasso solver:

```
using ParallelSparseRegression

dimensions, components, samples = 180, 100, 1500

dictionary = sprandn(dimensions, components, 0.1)
original_code = sprandn(components, samples, 0.1)
data = dictionary * original_code

# minimize \|data - dictionary*code\|_2^2 + \lambda\|code\|_1
# for variable code.
code = lasso(data, dictionary, 1)

# FIXME(Ariel): Is this the best way to check it found the real solution?
@assert(code = original_code)
```

Start with the test suite and reproduce the matrix example from the issue, checking how lasso currently handles data and code shaped as matrices. Add a regression test for the reported case and define completion as the solver accepting those inputs and recovering the original code as expected.

Written by the indexing model from the issue text.