Travis CI: Generate code coverage HTML and persistently store it.

This patch makes Travis CI generate Code Coverage reports in HTML format. Because Travis does not support persistent storage on its own, the generated HTML folder is transferred to phpseclib.bantux.org via rsync.

The latest reports are available at http://phpseclib.bantux.org/code_coverage/latest/ and a link to this page is added to the README of the repository.

The rsync transfer is protected by an RSA private key which is added as a file and protected by a passphrase. The passphrase is encrypted using the Travis CI public key of the phpseclib/phpseclib repository into a "secure environment variable". Because this variable is only available in the phpseclib/phpseclib repository, HTML reports can not be generated for pull requests and foreign repositories (which is a good idea). For pull requests #56 can be used which should be sufficient.

Btw, it was not possible to directly encrypt the RSA private key because of length limits of the used asymmetric encryption mechanism.