Merge pull request #6 from allanderek/dev

More .gitignore
Setup fix
Merge branch 'dev' of github.com:tdi/pyPEPA into dev
Logo added
Gitignore and manifest added
Fix dependencies
Mathematical expressions in rate definitions
Rate mathematical expressions
Merge branch 'cli' into dev
Fix where rate could not start with letter 'n'.

NOTE: we currently disallow infty and tau in rate definitions.
Small doc update
Merge branch 'dev' into cli
Prints in parser erased
Merge branch 'cli' into dev
Catching situations where target state is not defined.

Example:

P = (a,x).B; and B is not defined anywhere.
Got rid of exit(1) in lib, changed to generic Exceptions
Raise exception when variable not defined
Checking whether variable has been already defined
Merge branch 'cli' into dev
Updated history and setup