Remove dependency on 'feature' to avoid an undesired dependency on tcltk

As per the mailing list post from Kieran O'Neill:

```
However, at some stage flowCore gained functionality for performing
automated gating, and a dependency on the feature package from CRAN.
Feature depends on the tcltk C++ libraries, which in turn depend on
C++ libraries for a windowing system (X11 on *nix). This can make
compiling flowCore a massive pain on a headless server or compute
cluster. It can also be somewhat painful to more naive users on, say,
Ubuntu, who would have to install a number of development header
packages. I believe this would also be a problem on OSX.
```

This pull request inlines the required functions from `feature` and hence removes `tcltk` from the dependency tree.

We should make sure that we've adequately cited `feature`.