Starting work on RSA-based crypto stuff

as recommended (outdatedly?) in the Travis Pro docs at
http://about.travis-ci.org/docs/user/travis-pro/#How-can-I-encrypt-files-that-include-sensitive-data%3F
Implementing `travis-rsa-encrypt`
Implementing RSA decryption
Extra line makes the help text prettier [ci skip]
Accidental backtick there [ci skip]
Clarifying that `travis-rsa-decrypt` is meant to be run *on Travis*
Merge pull request #3 from modcloth-labs/rsa-crypto

Rsa crypto