Improve XDomainRequest implementation

This would help testing browser support.

removes ActiveXObject support, and improves detection of XDomainRequest.

Fixing config weirdness

Only use XDomainRequest <= IE 9

Fixing #171, don't fail on error responses with json payloads

got rid of the colon after Basic, test passed

Adding check for withCredentials in XHR adapter (#138)

Add support for baseURL parameter

Add support for HTTP Basic auth via Authorization header

Includes testing of the polyfills.

Only accept `username` and `password` as arguments

Use an `Authorization` header instead, which is a safer choice than URL embedded identities (aka `http://user:pass@example.com`). [Chrome 19 dropped support][chromium128323] for URL embedded identities in XMLHttpRequest for security reasons.

Added documentation note about how this will overwrite any existing `Authorization` header that the user may have set.

[chromium128323]: https://code.google.com/p/chromium/issues/detail?id=128323