Add bower support.

Hello,

This pull request adds bower support, I have already registered the package  ( https://github.com/openpgpjs/openpgpjs) as openpgpjs[1] to bower as well.


##### On Building /dist:
For security and transparency reasons, it is ideal that the /dist should be build in a public platform. I am not sure if it is possible to push the /dist from travis-ci to github or alternatives, something along the lines of Docker's [Trusted Builds](http://blog.docker.com/2013/11/introducing-trusted-builds/) would be nice.

[1] openpgp is taken by @luisivan, Do you think you can release the name as you don't seem to be keep it up to date with the upstream?