Fix/mspec params

Hi,

I recognized an error when passing more then one `*.dll` to the mspec task.

![image](https://cloud.githubusercontent.com/assets/444278/4233743/713a8398-39af-11e4-829e-1b6e10f69fe9.png)

At first it looks like that two assemblies could not be found.Certainly the mapping of the given `*.dlls` is treated as one concatenated string.

![image](https://cloud.githubusercontent.com/assets/444278/4233702/d7532348-39ae-11e4-997e-aa1f2061f3bb.png)

This leads into a wrong argument string for mspec.
```cmd
.. mspec-clr4.exe [options] -t "FirstSpec.dll,SecondSpec.dll"
```
It should be:
```cmd
.. mspec-clr4.exe [options] -t "src/FirstSpec.dll"  "src/SecondSpec.dll"
```

**What I have done**
- I fixed this issue by passing the src-parameter of the files object. 23d04b9
- I also added some specs to test the argument creation for mspec.
- For better testing I moved the `buildCommand`-Function into an own module called `mspec.command`.

Can you please check my modifications? You may want to update the npm-package.

Thanks in advance
Gregor