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 (mapped to 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
Merge pull request #48 from salcode/strict-mappings

change mappings to use strict mappings