Handle unary minus

Change to unary minus if the previous token is an operator, a left brace
or we are at the beginning of the expression/
Handle e^x exponentials

Bit of a hack here to handle exponentials of the form e^x rather than
exp(x).
Add unit tests for the standard unary minus and e^x syntax
Updates documentation. Adds authors file.
Bumps version to 0.0.8