Report the reason for bind failure

When an account's credentials are correct, but they still cannot
bind it is usually due to a locked account.

This shouldn't give any extra info to an attacker (they must
already know the username and password) but it does help
users who get locked out (too many incorrect passwords, for example)
to know what's wrong.
gitignore Gemfile.lock

The Gemfile.lock shouldn't be included in a gem-based repository
because it may artificially lock obsolete gems.

An example of this is rb-fsevent which at the version it was
locked at is incompatable with OS X 10.8

If something does need the version locked, specify it in the
.gemspec or Gemfile itself.
Removed @uri and construct_uri()

The attribute @uri and the method `consruct_uri()` were
not being used and prevented passing in failover hosts (not
yet supported in net-ldap, but hopefully soon).
Merge remote-tracking branch 'docwhat/gemfile_lock' into stable

* docwhat/gemfile_lock:
  gitignore Gemfile.lock
Merge remote-tracking branch 'docwhat/failure-reason' into stable

* docwhat/failure-reason:
  Report the reason for bind failure
Stablized to work with my ruby-net-ldap