Expose header of table view to allow setting search bar

[vitaliy-solarwinds]

This change will allow to set UISearchBar to LNZTreeView

For example:

    UISearchBar *searchBar = [[UISearchBar alloc] initWithFrame: CGRectMake(0, 0, CGRectGetWidth(self.treeView.frame), 44)];
    self.treeView.headerView = searchBar;

[codecov-io]

Codecov Report

Merging #33 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #33   +/-   ##
======================================
  Coverage    99.7%   99.7%           
======================================
  Files           4       4           
  Lines         334     334           
======================================
  Hits          333     333           
  Misses          1       1

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