Need to add exception and other code for missing dependencies

Need to add a Exception\DependencyException exception.

[ ] Add it to the source code
- [ ] Figure out where it is thrown and add it (taster I think)
[ ] Update docs
[ ] Unit test it
[ ] Need to add CSVelte\Exception\DependencyException class
[ ] Need to implement a CSVelte::assertDependenciesAvailable() method (or something similar)
[ ] Need to document the use of aforementioned exception class
[ ] Need to add unit test(s) for aforementioned exception class (for when Carbon or other third-party class is referenced but doesn't exist)
[ ] Need to document the use of exceptions in CSVelte in general
[ ] Need to unit test exception use in CSVelte in general (assertion methods and the like)
[ ] Make sure Carbon.php is the only file CSVelte will need from Carbon
[ ] Do some research and find out how modern PHP library developers handle dependency management for direct download/non-composer users

nozavroni / csvelte