updating the modelItem only on value change & fixed an issue in amount f...

...ormatters

Corresponding master pull request https://github.com/PrimeRevenue/tent/pull/407

<ul>
<li>Inside "saveEditedCell" method, added an "if" statement to update record only on value change.
<li>Update formatters to return '0' when it is zero instead of returning empty string.
</ul>

---

__General Requirements__
- [ ] Pull Request contains full description of changes
- [ ] Link to the Pivotal Tracker story
- [ ] Pivotal Tracker story has a reference to this Pull Request
- [ ] Rebased against latest master branch
- [ ] All comments from code review have been addressed
- [ ] There are no `-1` code reviews (can only be overridden by Kyle)

---

__Coding Standards__
- [ ] Style guidelines are followed, code has been formatted
- [ ] Code follows idiomatic language paradigms

---

__Testing__
- [ ] Test coverage has not decreased
- [ ] There is no performance degradation
- [ ] All appropriate automated tests are present
- [ ] Appropriate seed data exists


---

__Documentation__
- [ ] Major design decisions have been documented on the wiki
- [ ] Deployment impact (changes required for new services, configuration, etc.)
- [ ] documentation links from Pull Request

---

__UI/UX__
- [ ] applicable (UI/UX has been changed)
- [ ] reviewed for usability and design
- [ ] functions on 1024*768 browser size
- [ ] works on IE8 (presently only required for SCiCustomer)

***

__Defect__
- [ ] Feature or defect resolution has been manually tested and meets all requirements of the underlying story
- [ ] Seed data sufficient to satisfy testing resolution of defect
- [ ] Root cause analysis provided

---

__Database__
- [ ] schema changes reviewed by Architect
- [ ] Excessive SQL queries are not generated
- [ ] Slow SQL queries are not generated unintentionally
- [ ] No DB schema changes are backwards incompatible
- [ ] Database schema changes documented on wiki
- [ ] Migration provided

---

__Shared Project / Dependencies__
- [ ] Reviewed by Architect
- [ ] API changes to shared components documented on wiki
- [ ] 3rd-party library introduced:
- [ ] explanation for updating a project dependency (e.g. 'latest version fixes a bug')