Add engine_params parameter to flask_sqlalchemy.SQLAlchemy constructor

[RStatkevych]

One of the most discussed topic of issues is to add some additional parameters for passing to sqlalchemy.create_engine function (http://docs.sqlalchemy.org/en/latest/core/engines.html#sqlalchemy.create_engine). engine_params dictionary is passed 'as is' to sqlalchemy.create_engine, so now it allows to pass such parameters as isolation_level (#120 ), pool_pre_ping (#577 ) etc.

Example of usage:

from sqlalchemy.pool import StaticPool

db = SQLAlchemy(engine_params={'isolation_level': 'READ_UNCOMMITED', 'poolclass': StaticPool})

[davidism]

Duplicate of #487.