fix(closeManPageTab) : Removed bugs related to closing tabs

Changes in sys-view-model.js:
* Removed redundant checks surrounding the update of the currentActiveTabIndex to account for the event of closing a non-active tab.
* Changed the condition to update the activeTabIndex only for the case of closing a tab on the left of an active tab.
* Renamed previousTabIndex to newActiveTabIndex to better reflect the functionality change and initialised with currentActiveIndex(default case)

Changes in index.html:
* Prevented bubbling of click event on tab close button to prevent unexpected behavior.

Closes #44
Merge pull request #50 from anant-singh/bug-fix#44-ManPageTabs

fix(closeManPageTab) : Removed bugs related to closing tabs