Fix FunctionSymbol::compare()
Initial implementation of the Derivative operator

Tests will be added in subsequent commits.
Add a test for Derivative
Implement f(x).diff(x)
Add more tests
Implement second derivatives
Add Python wrappers
Implement get_symbol()
Add a test
Merge branch 'master' into derivative
Fix derivative printing, add tests
Add a test for f(x^2).diff(x)
Simplify s2b()

Now it should use just 1 allocation.
Use vector<RCP<Basic>> to avoid the s2b() hack
Improve the documentation
Test is_canonical()
Subs: std::vector<RCP<const Basic>>  ->  vec_basic

This is equivalent, but simplifies the code a bit.
Use simpler C++11 syntax for vector initialization
Use even simpler notation
Add few more tests