[Feature] Topology Checker Plugin.

Hi,
Glad to inform you the release of the 'topology checker plug-in'. Please test and provide feedback. I put a small video [here on youtube](http://www.youtube.com/watch?v=huhkTZkoKC8) to demonstrate how it works.

**What it does:**
QGis has the built-in topological editing feature which is great for creating new features without errors.But existing data errors and user induced errors are hard to find out. This plugin helps find out such errors through a list of rules.

The following rules are implemented as of now:

- must not have invalid geometries
- must not have dangles
- must not have duplicates
- must not have pseudos
- must not overlap
- must not have gaps
- Must not have multi-part geometries
- must not overlap with another layer
- points must be covered by segments
- Ponts must be covered by endpoints of line
- Line ends must be covered by Points
- Points must be inside polygon
- Polygon must contain point

I am working on adding a few more rules now.But these rules are the ones mostly used imo.The UI part surely needs some work.Help/suggesionts welcome.

**Wish to do further:**

- Move Configure Rules, Vaidate Extent and Validate All buttons from the dock window to to topology toolbar as icons.The existing dock window might be an obstruction for users(minimizing working area on canvas). Users only need the dock window to see what type of error is displayed. I might need help with this gui things as I am not so familiar with GUI. Also need icons for different themes(I have managed to put up something using inkscape temporarily though..)

- Option to save/load topology rules as xml.

- Option to display automatic fixes for different rules. For example, for 'must not have gaps' rule, an option could be given to user like 'create feature in gap'

- Option to specify a cluster tolerance, below which adjacent nodes/edges would be considered equal.