Feature: functions cbind and rbind (fix issue 787)

#### Summary:

Fix issue #787 by adding feature cbind and rbind.

Also fixed two minor issues: the function signatures test for to_matrix was incorrectly testing the inverse function and very minor fix to the documentation of Transposition Postfix Operator.

#### Intended Effect:

Create an easy way to concatenate matrices, vectors and row vectors. As I said on the issue, one of the application is to: "to address the problem of #266 in alternative way, and also provide an easy way to mix two vector to do predictive posterior when the model is such that inference on new data depends on old data, even when posterior estimated parameters are given like in the section 'Predictive Inference with a Gaussian Process'".

#### How to Verify:

Check the source code of the functions and of the tests.

#### Side Effects:

None.

#### Documentation:

Included.

#### Reviewer Suggestions:

Anyone.
typo: should be {rbind}