Use re.compile to speedup feature cleanups

[ashridh]

Fixes : #338 .

Each cleanup function first compiles the regex to a regex object, and then performs the sub on the text using this compiled regex.

[marco-c]

Compiling the regexes when the function is called won't give you any benefit.
For example in the case of bugs, for each comment you will call the function, compile the regex and then apply it.

The optimization we want to do is to compile regexes just once, and then use them many times.

Two possible ways to implement this:

1. Make the cleanup functions classes, like in bug_features, and compile the regex in their constructor
2. Make the compiled regexes global variables

[ashridh]

Yes, I realised this later. I think making them classes is better, as it'll maintain uniformity throughout cleanup functions and feature extractors.