Metasploit::Credential::Core association search

[MSP-10029](https://jira.tor.rapid7.com/secure/RapidBoard.jspa?rapidView=84&view=detail&selectedIssue=MSP-10029)

Search logins, private, public, and realm.  Searching tags is left to a Pro PR because Tags for credentials are only implemented in Pro in anticipation that `Mdm::Tag`s will be moved to Pro.

# Verification Steps

- [x] `bundle install`

## `rake spec`
- [ ] `rake spec`
- [ ] VERIFY no failures

# Post-merge Steps

Perform these steps prior to pushing to master or the build will be broke on master.

## Version
- [ ] Edit `lib/metasploit/credential/version.rb`
- [ ] Remove `PRERELEASE` and its comment as `PRERELEASE` is not defined on master.

## Gem build
- [ ] gem build *.gemspec
- [ ] VERIFY the gem has no '.pre' version suffix.

## RSpec
- [ ] `rake spec`
- [ ] VERIFY version examples pass without failures

## Commit & Push
- [ ] `git commit -a`
- [ ] `git push origin master`

# Release

Complete these steps on master

## `VERSION`

`PATCH` has already been incremented because adding new search associations is a compatible change.

## JRuby
- [ ] `rvm use jruby@metasploit_data_models`
- [ ] `rm Gemfile.lock`
- [ ] `bundle install`
- [ ] `rake release`

## MRI Ruby
- [ ] `rvm use ruby-1.9.3@metasploit_data_models`
- [ ] `rm Gemfile.lock`
- [ ] `bundle install`
- [ ] `rake release`

# Post Release

- [ ] Update https://github.com/rapid7/metasploit-framework/pull/3556 to use the released gem.
- [ ] Merge https://github.com/rapid7/metasploit-framework/pull/3556