Move amara import statement inside XML-based parsers, move pyyaml inside YamlConfig

Moving key library import statements to inside the class would make it an optional import. No XMLConfig, no amara. Regarding elementTree, see SESPRINGPYTHONPY-98, where we are replacing amara with elementTree.

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Here I see two issues:

• the amara dep is not resolved
• a minor one: if the same thing could be achieved via elementtree, it
  would be far better, since the latter module is part of the standard
  python dist since python 2.5. This would let springpython to run with
  one dep less.

Another one:
>>> from springpython.config import PythonConfig
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File
"build/bdist.macosx-10.5-i386/egg/springpython/config/_init_.py", line
16, in <module>
import logging
ImportError: No module named amara
>>>

This is another drawback of putting too many things into a single
_init_.py - why should I need amara (an xml toolkit) if I don't need
XMLConfig ? It is an unnecessary dep.