Taint the RStrings, not the buffer

Was causing data corruption!
Merge pull request #4 from nevir/patch-1

Taint the RStrings, not the buffer
Support building gumbo as a packaged library via mini_portile, as the
default behavior. Use --use-system-libraries to install via system libs.
Switch up the package naming so you can just require 'gumbo' to get
all the goodies.
Support for greater tag inspection
Add temporary gemspec so that the gem can be built via URL