Add AppVeyor support and fix tests on Windows

[macisamuele]

The goal of this PR is to ensure that bravado-core does properly support Windows platform.

The main changes needed are related to path 2 url conversions (check the fancy pathname2url and url2pathname functions).

Along with those fixes:

• I've ensured that paths building (especially during tests) are done via os.path and not hand-coding the paths
• Added appveyor configuration such that we could ensure that Windows is still supported NOTE: Having appveyor not happy with the tests is not a big blocker as the PRs could still be merged, but it will give us informations about potential regressions on the Windows platform

Appveyor tests are available on: https://ci.appveyor.com/project/macisamuele/bravado-core/builds/24347171

[coveralls]

Coverage increased (+0.1%) to 98.481% when pulling 6f6665522d209af54de92f3237a4999ca74d2db6 on macisamuele:maci-add-appveyor-support into 8fbb0cd0357ad5b4d296d4b175501a7a112593af on Yelp:master.