Merge pull request #55 from piwik/separate_auth_strategies

Refactored LdapAuth into separate classes and add new auth strategy more similar to pre-v3 plugin. Created three separate auth implementations, one for LDAP only, one for web server auth, and one for piwik auth w/ LDAP synchronization. New INI config options and tests added so users can select between implementations.
Display error message if LDAP server cannot be reached in integration tests.
Add user password field INI config option to specify where user password field exists in LDAP entries.