node ids shouldn't have to be ints

[fscottfoti]

It was recently discovered that node ids have to be ints with the current version. In practical terms this isn't a huge problem, but should probably be addressed at some point.

[kuanb]

It'd be helpful if there was a warning of some sort when Pandana takes in a dataframe that has strings in its index. It "sort of" does right now because you will see it has removed all n of n edges, but an additional warning if all 100% of edges are removed, perhaps suggesting the user check and make sure that indexes are not strings might be a helpful one line modification and hint that would improve the debugging experience.

[fscottfoti]

this is also fixed in #87