Partition doc fix: add </dl>
Add Depth[]
Add Operate[]
Add Through[]
Add IntegerDigits[]
Add a basic version of Variables[]

This is a start, but it could do with more work. It feels like
something SymPy could do, but I didn't find a function for it.
Add Complement[]
Add an example of IntegerDigits[n] with n < 0
add missing blank line
Simplify Operate by using get_int_value() instead of to_python()

get_int_value() handles conversion to Python and checking that the
input is the proper type. This means we only need one pattern.
Operate: improve message and add test case for n < 0
Use SymPy to improve Variables

This makes it match the first few examples in the Mathematica docs,
but it's still not complete. Add a note about this.
Merge pull request #148 from bnjones/mma-funcs

Add some structure-related functions
Implement ToCharacterCode and FromCharacterCode
implement Export and RegisterExport with Text format
fix WriteString to handle non-strings
add empty exprs WriteString test
refactor stream handling
implement BinaryRead and BinaryWrite
Implement BinaryRead types