Change ulpDiff to avoid extra memory swap

The compiler might be smart enough to figure this out, but why chance
it?
Avoid redundant comparison in Nearest

The first iteration of the loop was calculating the s[0] distance
again.  By starting on the index 1, we avoid the duplicated
calculations.
avoid redundant comparison in Min

The first index of min was getting compared twice.  By starting on the
second index, we can avoid it.
avoid redundant comparison in Max

The first index was getting compared twice in Max.  By starting the
loop on the second index, we can avoid the duplicate comparison.
remove statements that set zero value to zero

The ind variable is initialized at zero anyway, there is no need to set
it again.
add note about panic in LogSumExp

Because it calls Max, it can panic.
undo non-improvement to Max, Min, and Nearest

Turns out the doubled comparison is not a significant decrease in
performance compared to any alternative I could come up with that
avoids it.  Reverting.
Add missing periods to docs

Two public functions had missing periods at the end of sentences.
Fixed.
Merge pull request #11 from gonum/minor-code-improvements

Fix minor documentation and code issues.