change mappings to use strict mappings

In addition to changing to strict mappings, I limited the scope of some of the mappings
(specifically because by default they included visual mode but many of these functions do
not accept a range parameter).

Modes can be referenced here
http://vimdoc.sourceforge.net/htmldoc/map.html#:map-modes

Ack `<leader>f` mapped to only normal mode

CtrlP `<leader>p` mapped to only normal mode

NERDTree `<leader>n`  mapped to only normal mode

PDV `<leader>d` mapped to Normal, Visual+Select, Operator-pending because this will accept a range though it didn't work properly for me

`j` and `k` (perform gj and gk respectively) mapped only to normal mode (as previously set)

Adjust viewports `<leader>=` mapped to Normal, Visual+Select, Operator-pending (as previously set)

fixes #37