TST: Added unittest that runs all examples.
BUG: Removed SlippageModel from olmar example.

Fix crash due to 'delay' was no longer supported.
But removing SlippageModel override, since current configs
should be functionally equivalent to FixedSlippage.
BUG: Do not update last price if price is nan.
MAINT: Shortened time frame over which examples are run to speed up tests.

Minor fix pairtrade to now use record.
BLD: Add matplotlib to test requirements.

Example scripts use matplotlib, so add to requirements_dev.txt
so that CI will include matplotlib.
Merge addition of unit tests that exercise examples.

Also, multiple fixes to examples that the unit tests uncovered.