added plaintext oauth1 signature method.
added plaintext rspec tests.
finished up the missing signatures by adding in RSA-SHA1 support.
backported strict_encode64 method for ruby 1.8.7
moved base64 ruby 1.8 compatibility method into compat folder
remove unneeded reference to Base64 library
Merge pull request #26 from AnalogJ/master

added missing oauth1 signature methods