support general jdbc datasource

[Nicole00]

close #94

[Nicole00]

tested using mysql.
for mysql, the jdbc datasource configs should be:

      url:"jdbc:mysql://127.0.0.1:3306/test?useUnicode=true&characterEncoding=utf-8"
      driver:"com.mysql.cj.jdbc.Driver"
      user:root
      password:"12345"
      table:person
      sentence:"select * from person"
      partitionColumn:id    # optional
      lowerBound:1          # optional
      upperBound:5          # optional
      numPartitions:5       # optional
      fetchSize:2           # optional

[Nicole00]

When use jdbc to extract data from one database, please make sure that the corresponding driver jar has been located in ${SPARK_HOME}/jars. Or you will get exceptions like "java.lang.ClassNotFoundException: com.mysql.cj.jdbc.Driver"

[wey-gu]

Wow, this ideally will bring a bunch of new data sources!