Support selecting rows by clicking and meta-clicking the gutter

Closes https://github.com/atom/atom/issues/982.

This adds support for:

1. selecting entire rows by clicking the gutter. Previously, the whole row was selected only when the gutter was shift-clicked. See 8c36d26 for this change.

2. selecting multiple disjoint rows by meta-clicking (and dragging) over the gutter. Previously, this ability was not available. See 7f14965 for this change. 

:tv: time:

![clicky](https://cloud.githubusercontent.com/assets/38924/4110485/1017d520-31f3-11e4-8b6b-59670c5f1a77.gif)

cc @nathansobo @benogle for :microscope: :eyes:
I wasn't able to find a single function in `Editor` or `Selection` which allowed me to handle dragging in the way I expected the feature to work. Specifically, if wanted multiple overlapped selections to be merged into one, and clicking and dragging to always keep the initially clicked row completely selected. 

To make that happen, I ended up manually setting the selected region, triggering selection merging, and updating the selection reference after the merge. I added some comments which explain this -- I think they will be useful to Future Atom Hacker who looks at this part of code. :smile: Let me know if there's an easier way to do this, or if this needs more clarification in comments.