Add Exp method to Dense
Ensure receiver is not nil
Try to be tolerant of differance
Add Exper guarantee for *Dense
Let Exp work into a View
Elide the last multiply

benchmark            old ns/op      new ns/op      delta
BenchmarkExp10       69409          65405          -5.77%
BenchmarkExp100      10115387       9630431        -4.79%
BenchmarkExp1000     5259743720     4889078733     -7.05%
Merge pull request #46 from gonum/exp

Add Exp method to Dense