requirement: add pour_bottle? method.
Add PythonUtils class.
python_dependency: fixes, features, cleanup.

- PythonDependency now implies Python 2.7
- PythonDependency now blocks bottle installation if using non-system Python
- Use double-quotes
libxml2: use :python on 10.6.
pyqt: use PythonUtils.
pyqt5: use PythonUtils.
sip: use PythonUtils.
v8: just use PythonDependency as it implies 2.7.
requirements: remove unneeded Python27Dependency.

PythonDependency now implies this.
formula_installer: allow reqs to block bottles.
boost: use new requirement bottle blocking.
mkvtoolnix: check for Ruby 1.9.