Fix Invalid examples

1. String#encode should not fail if given an invalid code like '\x81'
This is because this problem has been resolved by #41.

2. Net::LDAP#search should return a boolean if return_result is false.
Adopt Travis-CI
Add status image to README
Update README.rdoc

Fix typo
Merge pull request #1 from satoryu/make_spec_green

Make spec green