Add option to set the default isolation level for the connection

[rossipedia]

We have a requirement that all queries use READ UNCOMMITTED by default. This PR adds an option, connectionIsolationLevel, that allows setting the isolation level in the initial sql batch that gets sent when a connection is opened (defaulting to READ COMMITTED so as not to change existing behavior).

Also, I noticed that using the single quoted block string was causing the newline to be trimmed, resulting in the wrong sql being sent

set textsize 2147483647set quoted_identifier on

instead of

set textsize 2147483647
set quoted_identifier on

Changing the batch to use a single double quoted block string and using coffee-script's interpolation fixes the issue.

Usage would be:

var tds = require('tedious');

var conn = new tds.Connection({
  ...
  options: {
    connectionIsolationLevel: tds.ISOLATION_LEVEL.READ_UNCOMMITTED
  }
});

[rossipedia]

Alternatively, the existing isolationLevel property could be re-used, although that would make it so all normal queries would have the same isolation level as explicit transactions.

[bretcope]

Another alternative is that it could be simply:

@config.options.connectionIsolationLevel ||= @config.options.isolationLevel

And the documentation could read something like:

Each new connection will be opened with this isolation level by default. Defaults to the value of options.isolationLevel.

The only issue with this is that it could change behavior if someone was intentionally expecting a different isolation level for general queries vs. transactions. It seems sensible to expect the same default for both, but either way will address our needs.

Obviously the docs for github.io should definitely be updated when this gets accepted.