Implement ‘-’, ‘+’, and ‘enter’ keys

implemented as `Motions.MoveToFirstCharacterOfLineUp` and `Motions.MoveToFirstCharacterOfLineDown`

I simply compose either of the functions `MoveUp` or `MoveDown` with `MoveToFirstCharacterOfLine`.

Includes links to the Vim documentation for these motions in the docs folder

The enter key was already mapped to `Motions.MoveDown`, but its functionality is actually more complicated than that.
Add tests for ‘-’ (failing, partly due to bug in ‘k’)

And add a stub for the tests for ‘+’ and ‘enter’.
Fix bug in - and +: don’t move to first line char when selecting

I found this bug because a test had the cursor position [0, 0] instead of [0, 4].
Comment out tests for `-` that are failing due to `k`

I don’t want to delete these tests because they will be useful to have when `k` (and correspondingly `j`) is fixed. Otherwise, they would have to be rewritten at that time, or they would not get written and `-` and `+` would not be as well-tested.

The comments explaining why these tests are commented out and saying when they can be re-enabled should protect these tests from being left disabled when they shouldn’t be.
Merge branch 'refs/heads/master' into implement-minus-plus-enter

Conflicts:
	lib/vim-state.coffee
	lib/motions/general-motions.coffee
Add tests for + and enter, mirroring the tests for -

As requested [in the pull request discussion](https://github.com/atom/vim-mode/pull/277#issuecomment-47765173), I am adding the tests for + even though some of the copied tests are disabled.

These tests are all the same as the ones for -, except that the starting line numbers are X from the bottom instead of X from the top, and in the “abc” sample text the order of the lines is reversed.

I also changed a previous test for - so that its cursor column is less remarkable, and won’t fail due to edge cases that that particular test is not aiming to test. The cursor column in that test really is now in the middle of the line, instead of on the last character.
Add tests for enter, checking equivalence to +

I added tests for enter too, for the heck of it. To avoid duplication, the tests merely compare the results of + and of enter and make sure that they are the same.

I could have tested equivalence of enter and + in more situations, but I think it’s likely that if they are the same in the situation tested, they will be the same anywhere. It would be an unlikely mistake for someone to change the keybindings of enter and + to different functions, and then edit only one of the functions in such a way that the results are only sometimes different.

I also changed some single quotes to double quotes for consistency.
Comment out only the failing assertions in the - and + tests

The behavior of `j` and `k` have changed since the tests were written. The commented-out tests still fail, but not as badly. The cursor column is still wrong, but the cursor row and the text in the editor are now correct.

Thus, so that the text in the editor can be automatically tested, the tests have been uncommented, and only the screen position check has been commented out.
Merge pull request #277 from roryokane/implement-minus-plus-enter

Implement ‘-’, ‘+’, and ‘enter’ keys