Feat/parameterized sql queries

[timsaucer]

Which issue does this PR close?

Closes #513

Rationale for this change

Users would like to use DataFrames as a parameter inside an SQL query. With this change, you can do the following:

from datafusion import SessionContext
ctx = SessionContext()
df_customer = ctx.read_parquet("examples/tpch/data/customer.parquet")
ctx.sql("select c_custkey, c_name from {df}", df=df_customer)

The string {df} in the query will be replaced with the SQL equivalent of the logical plan of the DataFrame.

What changes are included in this PR?

All of the read_parquet, read_avro, read_json, and read_csv have been changed to call register_ with a generated table name. This table name is the file name. If a table already exists with that file name, a generated UUID is used instead.

One unit test is included.

Are there any user-facing changes?

There is an addition of an optional table name to each of the read_ functions above, but it is a non breaking change for the users.

[MrPowers]

This user interface looks nice 😎