Assorted fixes/improvements

[thchr]

Hi @wildart

I went ahead and vendored this package as a git subtree in my own package.
In doing that, I had a quick look at the code and wanted to change a few things: I'm not sure if you're interested in me upstreaming anything, but in case you were, I figured I might as well submit a PR (if you are not interested in maintaining this much, PRs etc., no problem - I just wanted to submit it in case you were).

The main things this does is:

• Previously, the SNF field of Smith was an AbstractVector{P}; I added a V parametric type to Smith so that this could be concretely typed instead.
• A few simplifications, e.g. in diagm.
• Slightly more info when printing a Smith struct, and also overload show(::IO, ::MIME"text/plain", ...) instead of just show(::IO, ...).
• A doc string for smith.
• Some whitespace/alignment cleanup.

I had originally wanted to remove the dependency on SparseArrays, but it seems that it is not so easy to do generic programming with sparse vs. dense matrices as I had thought.

[coveralls]

Coverage decreased (-2.4%) to 92.67% when pulling b752789 on thchr:master into 0b61553 on wildart:master.

[thchr]

There was a test failure on v1.1 due to diagm and spdiagm apparently missing some methods there, relative to more recent versions; just rolled back to what you had before there then.