In-place mocking

Temporarily overwrite the memory that stores all information about a function to be mocked (formals, body, environment, attributes). Now, also mocking imported functions works as expected.

Fixes #171.

Doesn't include #175 anymore.
The following reliably hangs (!) on my Ubuntu x86-64 with R 3.1.1:

```
Rscript -e 'for (i in 1:100) devtools::test(filter = "mock")'
```