Check unmodified mappings for g:NERDTreeNoOverride

Note: this fixes the issues mentioned in: 208a762.

- The `g:NERDNoOverride` variable is case sensitive
 - This is specific to the plugin and doesn't need to be changed
- The strings in `g:NERDNoOverride` are case sensitive and must be
  specified exactly
  - This has been fixed and noted in the docs.  Mappings to ignore are
    lowercase strings.
- Keys like '<c-k>' must be specified as '<lt>C-k>' for the time being.
  - This has been fixed