Support for Application-only request #8

If you request a bearer-token (app-only) it will be used instead.

Merged the app-only token functionality from
the pull-request on the abrahams tree (see issue #8 )
Coding-style might not be optimal compatible.
- put an api-url detecting in getUrl()
- removed OAuth key requirement (moved to getOauthString())
- buildRequestHeader() will detect bearer-token and set accordingly
- example_app-only.php added

Also fixes a bug, where if you POST something, a subsequent GET
will get confused by dangling parameters and method setting
- reset $this->method and $this->postParams after a request