Add InputStream#ungetc

Works like IO#ungetc
Use String#reverse in test for 1.9 compatibility
Use string concatenation instead of String#prepend

For < Ruby 1.9.3 compatibility
Declare utf-8 encoding for the unicode tests.

Will fail without this as there are utf-8 characters in the file.
Rename the unicode tests file so it actually runs.
Merge pull request #162 from hainesr/run-unicode-tests

Actually run the unicode tests
Merge pull request #141 from zacstewart/master

Add InputStream#ungetc