Added is_literal, modified literal_symbol
Added tests
Handled AttributeError
Merge pull request #1865 from srjoglekar246/literal

Added is_literal, modified literal_symbol
Added rank() to matrices

Gives the rank of a matrix via rref.
Add Benjamin Fishbein to AUTHORS/aboutus. Welcome to SymPy!
Merge branch 'master' of https://github.com/fishbeinb/sympy