Refactor metadata implementation

- Support individual key/value setting without wiping
- Remove now-redundant makeRequest/transfromReq hooks from StripeMethod
- Bulk-setting metadata is now done by 1 full-reset req followed by the
new-set instead of finding individual props to reset (was pointles).
add ability to get a single metadata key via getMetadata
Fix URL path issue in Windows. See issue #68

We were using path.join for joining url paths, which obviously
failed in Windows since url paths don't work like win fs paths.
There is no equiv url.join (url.resolve is a little different)
so for now this is the simplest fix to get it to work. The paths
used are within our control via the resource modules so I
think this is ok.
Add update method to charges resource
Add update method to transfers resource
future provision
clean-up metafiles, incl. improved readme and bump in preparation for 2.1.0