Add sql helper function

Currently the only function is loadSql() which is hard-coded to read in
a sql file located at inst/sql/sql_server/. It is a stripped down
version of what is found in OHDSI/SqlRender.
Add Connection class

This class is intended to hide most of the details of a database
connection. It only has four methods: connect, disconnect, execute and
executeforresult. The sql queries one provides to execute are currently
assumed to be in Sql Server syntax and is translated to the connection's
syntax by the execute method. This means that if one is using
parametrized sql, that sql should be rendered before the execute method
is used.
Add Hdps class

The Hdps class is meant to be a wrapper around the Connection class
which handles the model-specific portions of the HDPS algorithm.
Currently it has the capability to build cohorts as well as check the
size of the cohorts.
Add README file