go for 2x2 complex matrices

Complex matrices 2x2 (a.k.a. **m2c**) are a wonderful land to explore.
They are matrices of the form

```
| a b |
| c d |
```

Where a, b, c, d are complex numbers. I found Golang as the perfect
language to implement them since it has `complex128`

data type, yeah!

First of all import `m2c`

.

```
import "m2c"
```

Get the identity matrix

```
var id = m2c.I()
```

Multiply two matrices.

```
var a = m2c.Matrix{1, 0, 0, 1}
var b = m2c.Matrix{1, 1+i, 0, 1-i}
var c = m2c.Mul(a, b)
```

Multiply two matrices.

```
var a = m2c.Matrix{2i, 0, 0, 1}
var invA, err = m2c.Inv(a)
```