Fix a typo in numbertheory.py, fix some docstrings.

Some improper formatting in docstrings was preventing
a few symbols from showing up in the documentation.
Fix IntegerExponent erroring on symbolic input.

IntegerExponent's pattern was changed to evaluate
only on [_Integer, _Integer] input.
Fix IntegerExponent erroring on symbolic input.

IntegerExponent's pattern was changed to evaluate
only on [_Integer, _Integer] input.
Add IntegerExponent test to verify symbolic behavior.
Merge pull request #111 from sn6uv/RecursionLimit

correct $RecursionLimit tests
Fix minor typos and pare down some noisy function tests in the docs.
Merge pull request #108 from freshwater/typofix

Fix a typo in numbertheory.py, fix some docstrings.
implement RegisterImport
add ExampleData1.txt example data format
begin rewriting Import
add mathics/autoload/format/Text/Import.m
Implement Conditional and Post Importers for RegisterImport
cleanup of RegisterImport
cleanup Import
implement conditionals for Import
Post importers are actually optional and occur seperately to defalt/conditional importers
cleanup Import tests
Implement Text Importer
fix bug in ReadList: graceful fail
implement StringSplit