Support virtual hosted-style requests

Supports making only simple GET, PUT and HEAD requests for now. Bu-
cket manipulation will probably fail.

This requires that the base URL is provided in hosted-style.
Include sub-resource parameters in the string to sign
Do not add '=' if parameter has no value
Allow x-amz-security-token header when applicable
Merge pull request #3 from shahriman/develop

[RFC] Minor improvements to make the library a little more useful