Summary: Removes get_cursor method and tests

Test Plan:
- run specs
- run system tests

Reviewers: khamidou

Reviewed By: khamidou

Subscribers: mg, spang

Differential Revision: https://phab.nylas.com/D2360

The limit and offset filters, which were set correctly in the `where`
function, were being overriden in the `get_model_collection` function
by the offset and limit parameters passed to it (with defaults). The
function now checks if limit or offset are already present in the list
of filters before applying its own.

Close issue #66.

Add reply_to field to Message class

Previously, limit and offset could be specified in `@filters` (which
were usually set inside a higher-level function such as `where` or
`range`) and also as function parameters to the lower-level function
`get_model_collection`. The decision as to whether or not the passed
parameters would override those already present in `filters` was
confusing and has been removed in favor of only using the `@filters`
method.

`delta_stream` is a blocking call, and previously had no good way to run
in parallel in order to stream several accounts at once.

EventMachine's run loop was removed from the library so that SDK
implementers can create multiple streams inside their own EventMachine
run block.

`delta_stream` must be wrapped in an `EventMachine.run` block in order
to function, whereas before it could be called plain. Two tests have
been refactored to reflect this, and documentation has been updated.

Also includes small optimization of the `while` function.

Refactor `delta_stream` to allow multiple streams

Allow limit and offset filters for collections