Adding config option for specifying certificate value instead of path to...

... certificate

If certificate_value is passed to configure, it uses that string to create the RSA key. It is expected (but not checked) that this string is of a valid format with newline characters in appropriate places, e.g.
```
-----BEGIN RSA PRIVATE KEY-----
asdf123FOO$BAR
...
-----END RSA PRIVATE KEY-----
```
An incorrectly formatted string will raise `OpenSSL::PKey::RSAError: Neither PUB key nor PRIV key:: nested asn1 error`. Maybe there will be a feature that if certificate_value raises that, to try certificate_path instead.


Other changes:
aggcat/configurable keys are DRY'd

There was an error about rake being required twice in gemfile. Looks like it is only needed for tests, so moved to test group and removed from gemspec. see http://docs.travis-ci.com/user/languages/ruby/#Default-Test-Script