Unit conversions and more Unit tests for RevitNodes

### Code changes ###

This pull request provides unit conversions in a few of the locations in `RevitNodes` where they were lacking.

Similar to the previous PR for `ReferencePoint` units, there are other `Element` creation methods that use raw numeric values to create and Element.  Previously, these methods did not do proper unit conversions.  With this pull request, they do.

Further, there were a few methods of extracting `Geometr`y or numeric values from `Elements` (e.g. `Level.Elevation`) that did not properly convert units.  With this pull request, they do.

### Unit tests ###

This pull request includes new unit tests and improvements to existing unit tests to check the "internal" numeric values - which are actually the Revit internal units in feet.  These tests are only added for the new code changes.  

### Additional changes ###

This pull request also removes the `AssertShouldBeApproximately` extension method from `RevitNodes`.  This method is for comparing XYZ's, Vectors, Points, at a user defined tolerance.  It is replaced with `ShouldBeApproximately`.  This is the original name for the method, which had been refactored to the former and replaced with a method that did not perform an assert.  Go figure.  

### Results ###

For what it's worth, all of the newly added tests run 100%.

![tests](https://cloud.githubusercontent.com/assets/916345/3475091/4cc148ca-02ec-11e4-9ed6-c6a51db3d696.PNG)


@ikeough @lukechurch