Implementing lexema_cat and changing lexema_append

lexema_cat function do a similar work that lexema_append does. So,
I changed lexema_append to depend on this new one. Also, I inserted
a new test just to make sure that the input data of lexema_cat is
not release.
Removing Expression mock

The Expression mock was divided in Expression and ConstantExpression
class. I could make stuff like ArithExpression, RelationalExpression
or BitExpression, however I can't see a good use now.
Adding coverage info

I'm putting a way to generate coverage info about the tests that I
ran. The python file is another project called gcovr, I'll setup
Drone.IO to run this and check if we had lines with something like
this expression: "{digits}+(,|\-{digits}+)". This will detected
that a file has more than one line not tested. This sample rule
will keep the coverage over 95%, I hope.