Enable LDAP connection string building in rimpala.connect()

[bkvarda]

I wanted to make building an LDAP connection string possible without changing anything in your Java code. Now if someone uses rimpala.connect(user="someuser",password="somepassword",db="somedb"), the "principal" string when user and password are defined will look like "somedb;user=someuser;password=somepassword". In RImpala.java, the appropriate connection string for LDAP will be created.

CONNECTION_URL = "jdbc:hive2://" + IP + ':' + port + "/;" + principal;

In this example the full string would become:
"jdbc:hive2://localhost:21050/somedb;user=someuser;password=somepassword"

This follows the appropriate convention for an LDAP connection string as shown in the Impala JDBC docs:
"jdbc:hive2://myhost.example.com:21050/test_db;user=fred;password=xyz123"

http://www.cloudera.com/documentation/enterprise/5-2-x/topics/impala_jdbc.html

If a user parameter is not entered in rimpala.connect(), the existing logic for building the "principal" string still takes place.