Update asv config + fix some broken benchmarks
- Enable platform-dependent config in asv (needs asv git version for it to do something)
- Enable wheel cache in asv (in asv git version)
- Fix a few easily fixed broken benchmarks

can you rebase this. we can prob throw this in 0.18.0; it doesn't affect user code.

Can you run vs 0.17.1 and post anything interesting?
 Rebased.
Don't have time to run the full bench suite right now, but can be done via
`asv continuous 6c30cbecf8 master`
 benchmarks: https://gist.github.com/pv/8ff97c3f6faa62fd6a5e --- not sure if all of those are real, or fluctuations (laptop under load)
 I think we need to update the perf docs in contributing?
 Not sure --- they already instruct to install the asv git version.
 Added explanation on how to choose between conda / virtualenv / existing environment to contributer docs.
 thanks @pv 

pls check the docs when built (prob a few hours) and if necessary issue a follow-up
 should these be ignored as well?

```
[jreback-~/pandas] git st
On branch master
Your branch is up-to-date with 'origin/master'.
Untracked files:
  (use "git add <file>..." to include in what will be committed)

        asv_bench/.nojekyll
        asv_bench/results.orig/
        asv_bench/run_versions.sh

nothing added to commit but untracked files present (use "git add" to track)
```
 sorry....these look like mine. nvm!
