Add easy way of adding Authorization header

Resolves #84
Closes #85
Allow pass Authorization header as is
Merge pull request #86 from tarcieri/feature/auth-header

Add an easy way of adding Authorization header
Use strict_encode64
Prove that encoded credentials are WS-free
Add Ruby 1.8.7 #strict_encode64 support
Bump up YARDstick threshold
Add info about proxy support

Closes #83
Merge pull request #88 from tarcieri/fix/auth-encoding

Strict encode authorization header value