Removed slash from URI helper doc

In my current projects I don't see '/controller/method' but 'controller/method' when calling uri_string().
Merge pull request #2186 from 12k/patch-1

Removed slash from URI helper doc
Move db_select() call from CI_DB_driver::initialize() to db_connect()
so that it's only called by drivers that need it ('mysql', 'mssql').

As proposed in issue #2187.