fix(dropdown): use button elements instead of links

This fixes accessibility and cursor display issues.
Part of #142

closes #490

demo/src/app/components/dropdown/demos/basic/dropdown-basic.html

@@ -1,8 +1,8 @@
 <div ngbDropdown>
     <button class="btn btn-outline-primary" id="dropdownMenu1" ngbDropdownToggle>Toggle dropdown</button>
     <div class="dropdown-menu" aria-labelledby="dropdownMenu1">
-   	    <a class="dropdown-item">Action - 1</a>
-        <a class="dropdown-item">Another Action</a>
-        <a class="dropdown-item">Something else is here</a>
+   	    <button class="dropdown-item">Action - 1</button>
+        <button class="dropdown-item">Another Action</button>
+        <button class="dropdown-item">Something else is here</button>
     </div>
 </div>

src/typeahead/test-common.ts

@@ -6,19 +6,18 @@ function normalizeText(txt: string): string {
 }
 
 export function getWindowLinks(element: DebugElement): DebugElement[] {
-  return Array.from(element.queryAll(By.css('a')));
+  return Array.from(element.queryAll(By.css('button.dropdown-item')));
 }
 
 export function expectResults(nativeEl: HTMLElement, resultsDef: string[]): void {
-  const pages = nativeEl.querySelectorAll('a');
+  const pages = nativeEl.querySelectorAll('button.dropdown-item');
 
   expect(pages.length).toEqual(resultsDef.length);
 
   for (let i = 0; i < resultsDef.length; i++) {
     let resultDef = resultsDef[i];
     let classIndicator = resultDef.charAt(0);
 
-    expect(pages[i]).toHaveCssClass('dropdown-item');
     if (classIndicator === '+') {
       expect(pages[i]).toHaveCssClass('active');
       expect(normalizeText(pages[i].textContent)).toEqual(resultDef.substr(1));

src/typeahead/typeahead-window.ts

@@ -16,11 +16,11 @@ export class ResultTplCtx {
       <ngb-highlight [result]="formatter(result)" [term]="term"></ngb-highlight>
     </template>
     <template ngFor [ngForOf]="results" let-result let-idx="index">
-      <a class="dropdown-item" [class.active]="idx === _activeIdx" 
+      <button class="dropdown-item" [class.active]="idx === _activeIdx" 
         (mouseenter)="markActive(idx)" 
         (click)="select(result)">
           <template [ngTemplateOutlet]="resultTemplate || rt" [ngOutletContext]="_prepareTplCtx(result)"></template>
-      </a>
+      </button>
     </template>
   `,
   directives: [NgbHighlight]