added a drop tables sql script in order to clean out mysql tables

[mahtin]

In order to get sync to work sometimes, I find it useful to delete all the tables in the database and run peeringdb sync again. This the sql file I use.

[codecov-io]

Current coverage is 0.00%

Merging #7 into master will not affect coverage as of 1a200af

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[grizz]

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