Initial untested get code
Adds getMany methods

First draft complete. No verification that it works yet, and there are
many problems with it:

 * Type should be an optional parameter
 * It would be nice to generic type the response on the type provided
 * URL handling needs to be improved

Really this should cater for the easiest use. So URLs provided as
strings should just gain a 'callback' parameter. It should still be
possible to use a function to generate the url though.
Change internal method

Change _get_as to _to_type to better reflect intent. It also releases
the js.Proxy object now, to simplify the handlers.
Fold the getAs/getManyAs methods into get/getMany

Using the transform and then operation afterwards rather than changing
in place means that different calls to the many methods can provide
different types (for the same stream). It also makes the code simpler,
and only has the overhead of retaining the js.Proxy data for an
additional method call.
Merge branch 'feature/fix-type' into develop
Rename variable
Add method to handle the default callback way

Just adding a callback= query parameter is the normal way for jsonp.
Switch to using uri generators or just uris

This way should make it easy to get data and still make it possible to
do advanced stuff.
Missing the uri / generator is now an exception

Want to fail fast when people call it the wrong way.
Remove now-wrong generic type
Merge branch 'develop' into feature/uri-handling

Conflicts:
	lib/jsonp.dart
Merge branch 'feature/uri-handling' into develop
Defensive coding

Make the _get function a little more prepared. It may be that the other
checks go and this is the only one that remains - it is here that it
matters.
Flip methods
Formatting
Add draft documentation
Fix string concatenation
Fix updating of url
Fix construction of instance
Better fix for construction

Doesn't return the InstanceMirror