Fix warnings and small mistakes in several lexers

[brodock]

When running tests for a gem that depends on rouge I've noticed that there are a bunch of relatively easy to fix warnings popping up during the test execution.

I've split into separate commits each warning / fix per file required and added an explanation on each commit message of what was the problem.

When evaluating each regular expression issue, I've used Ruby 2.0.0 Regexp documentation as base.

---

To reproduce the warnings above, I've run specs with -W2 option enabled:

RUBYOPT='-W2' rake check:specs